It could be your diet, if you have recently gone vegetarian. However, as a vegetarian, you probably already consume a lot of things that contribute to healthy hair such as soy, wheat germ and brewer's yeast. You may want to add more protein to your diet.
Are you on hormonal birth control? Sometimes this can cause hair loss.
It's hard to define exactly when it is too much hair to lose. Basically, if it is coming out in clumps or is noticeably thinner, you are losing too much hair.
Althought you are taking supplements, they may not be enough. Many of the same vitamins that strengthen hair are found in prenatal vitamins. So, some women take those. You get all the normal vitamins you need with an extra boost of the ones that help combat hair loss. However, you should be careful that you don't overdose on any vitamin. Check the levels first before you add them to your daily routine.
Good luck.