I suggest you drink lots of water and take biotin pills to make your hair stronger. I also recommend you keep your hair moisturized on a regular basis. Weaves and braids can be very harmful to hair if they are styled very tightly. I also suggest you stop styling your hair that way. I am going to assume that when you do put in weaves and braids that you leave them in for an extended period of time. As one of the other commenters stated, we do lose about 100-150 hairs a day, so it's probably build up. By the way, when you do wear weaves and braids, I hope your keeping your hair moisturized. I've seen a few YouTube videos of women shampoo and conditioning their braids. You should try it.
If you wear protective styles all the time, are you ever detangling? Does this happen when you finally detangle? If this happens after you haven't detangled in a long time, it is completely normal. We lose about 100-150 hairs a day. If you aren't detangling and your hair is in a protective style, these hairs aren't being removed, so when you handle your hair, your hands are manually removing a whole bunch of them at once and they can definitely knot up.
If you are detangling frequently and your hair is constantly falling out, you need to see a doctor. I myself have hypothyroidism which causes hair loss. It can be various things, but I know that if it's thyroid related, it can be dangerous if untreated. Don't panic, just pay attention and try to figure out if it's dead hair that is falling out.