Quite a few medications have the side effect of making the hair fall out. There is a website devoted to it called Drugs that can cause hair loss- women's hair loss project. The major culprits seem to be hormones and anti-depressants. If you started taking a new medication around the time the heavy hair loss started, that's probably the culprit.

I did not start taking a drug but my hair is slowing on the fall out. I am taking prenatal vitamins, and using foam rogaine. Its still thin but hopefully the worst is over. I have to clip in a hair extension to make it look normal and use toppix to fill in the really thin parts...thankfully its more of a volume problem than a scalp issue.

the clip-ins will stress your hair and could cause more loss. ironic isn't it? but really they should be used only occasionally (same with teasing the hair).