Prenatals

I have been taking prenatal vitamins for just over a month because I heard they can help when TTC. I've never had an issue with them but the last few days I've been getting a very upset stomach after taking them. I thought it was just because I did not have enough food in my stomach so I started taking them with a full meal. Today I took my vitamin with my lunch and I still feel horrible about 15 minutes later... Does anyone have any experience with this or any idea why it's happening? I am currently 5 days late for AF but all tests(one being this morning) are negative.... Any input?