My mom eats then forgets I fed her, and insists that I feed her again. Any advice?

Take a cell phone photo. A picture is worth 1000 words.

Ask doctor to evaluate her for palliative care/hospice care. After this, he should be able to have her evaluated.. This means that a nurse will come to your home and take vital signs etc.
With Hospice, they checked her "weight" by the diameter of her upper arms, above the elbow. This is an indication if fat is building up, or going down.. as far as I recall.
If you take pictures, take selfies with her holding a milkshake or something with you, make it a fun and happy picture so she doesn't get upset about it. Play music,,.. You can always mix her ensure with a milkshake to make it taste better. That is what my friend and I did with our moms when they were roommates at a board n care. We made it fun, lively and happy..:)
Is she able to participate part time at an adult day care a couple days a week? Insurance may help pay for it.. I don't recall. I don't think they did with my mom, but that was a number of years ago... Senior activities during the weekday, to give you a break, and mom something new to do to activate the brain differently.. New people, activities, daily news, where in the World are we Talking about..Geography... puzzles,etc... you get it... IF YOU CAN ENGAGE HER IN OUTSIDE ACTIVITIES A FEW DAYS A WEEK, WILL HELP TOO.
So when she comes home to you, she will be happy to see you...and she knows you are doing the best you can... Sometimes, all of us need a break.. :)
Glad she is around for you to get concerned... :)
Glad is able to talk, and make conversations... When that stops,,, IT IS REALLY SAD...:(
On that note, please be patient with mom, do not correct her if she doesn't get a name right, or a date right, or what she had for breakfast... When she stops talking, that will be very devastating. I asked mom what was my name, .. her sister's name popped up... I corrected her.. she stopped talking... It hurts... then you are the master of facial expressions, you think... happy? yes... frustrated/confused? looks like it....crying...it hurts, or she is sad......or scared...
I am glad your mom is well taken care of... you are doing a good job! Keep it up. You are all she has, make it fun, enjoyable and happy... You Both Deserve It!

I have been following this trail of comments but don't see anyone mention about what to do about Mom, who is 87 and lives with me since Dad passed away last August, is foraging (I was calling it Grazing) on her own (to the refrigerator every 10 minutes in the afternoon when she is awake, and ALWAYS right after she wakes up from napping, which is all day long.) She doesn't ask for anything just keeps going back, even when I am in the middle of preparing dinner. I keep yogurts, fruit cups, Ensure drinks etc. there, and she keeps making Jam sandwiches. She gets defensive when I remind her what she just ate for lunch. Her non refrigerated "go to" is cereal. Several times a day if I let her.

How do you deal with this when they are mobile on their own? I feel awful trying to stop her from another serving of whatever. "But I want it!" She just dumped the rotissierrie chicken on the floor rumaging in the fridge. I keep the toaster under the counter, unplug the microwave, (she burned bread in it a few weeks ago), and took the knobs off the stove.

I know it's the disease, but it makes me just about quiver with frustration. She was doing this before, but dealing with it on an hourly basis is awful. I Suppose it could be a lot worse, and probably will be. I keep a regular journal, and started to do a food journal for the particularly bad days. Will run it by her Doctor this week.

Any other helpful hints?

Your mom has lost the ability to reason. Don't get yourself upset when she says strange things. Trying to convince her that she has eaten will be a futile effort. It is the disease. If she continues to complain about you, her Dr. will see the pattern. My mom (aggressively) tells me not to burn her whenever I turn the water on for her shower. I have never burned her, it's just something she says. I ignore it. Give your mom small portions, put each addition serving on a different plate, leave the plates in front of her. If she has set activities, plan her meals shortly before them so that there is a time limit.