Is my Mom developing Alzheimer's? Please help

Hello everyone I'm writing today in desperate need to know weather my 67 year old mom is starting to develop Alzheimer's. She believes that my brother's EX (she hates her) wife is coming into the house and moving things around. She got a new alarm system, she locks up every day, and she believes there are hidden cameras in the house watching and that's how my mother feels she's able to get into the house. Me not knowing or understanding what Alzheimer's was I told her there is no one coming in the house and she still believes things are being moved around. She says to me that my brothers EX wife is moving things around just to let her (my mother) know that she was there. Again I did not think nothing of it. Then one morning came and I forgot to lock the door and she screamed at me and right then that's when I looked up information on Alzheimer's. When I asked her about it she screamed and said she's not crazy. Don't get me wrong, my mom still goes to work, does normal things in the home, and she doesn't forget things. But she somehow believes that someone is coming into the house. I sat her down yesterday and told her my concerns and she said to me that she gets full check ups from her doctors. She still drives and do eventing else well, but she takes care of everything (I'm crying). She's the strongest lady I've ever known and to think that she may be getting ill scares me and sadden's me. She told me the only thing that would make her upset is if we (her sons) believe's that she was crazy. I'm new to understanding changes In age because I myself am very busy, but for the last few days reading about alzimers I started to sit back and watch her. I notice that she sings or hums in house a lot more, she forgot I gave her a glass of water the other night, small things like that. For the past 4 days (crying my eyes out) my mind is going crazy not knowing what to believe because she said she Go's to her doctors to check herself and say she's fine, She has high blood pressure and that's all I know of so far. Also I'm the one she talks to about most times about things that go on In the house, and now I think she thinks I'm no longer the person she can trust (crying). Please understand that my mom is my best friend who i respect and love dearly and it's kinda breaking me down mentally. I'm not sure what's going on. I'm confused. Is this the starting of Alzheimer's? She been locking doors and thinking people are coming in for the past 4 or 5 mounths. Is this something to be concerned about? Please God not my mother ......thank you