Always being so negative

Hi everyone, (Sorry for any grammatical errors, English is not my native language) 

 So my girlfriend has ADHD and we've been together for around 9 months now. Altough I am happy with her, there are a few things which really bother me and I'm not sure if it's her ADHD, or the way she is. 


Something that happened today is a good example of a thing that's really annoying me:

She started to became a veganist a few weeks ago and it's all going fine, except that she didn't took meat replacements so she has a lack of a lot of vital vitamins which caused her to feel sick at the moment. So after days she found out her sickness came from the lack of vitamins, but that was it. She didn't do anything about it so again a few days later I told her she really had to take vitamin boosts. Her reply to that was that taking those will make the human body lazy so that she doesn't want to, after that I only told her she should take them because it's almost the only way for her to feel better. 

So today in the morning she said she feels so bad so I told her that it'll be just a few more days and then she'll feel fine again because then the vitamin boosts will start to work. Her reply was like "vitamin boosts?" so it seemed like she forgot about them, so I told her "Yes those pills your mom bought you, please take them they will help you and if you only take them for like 2 weeks, your body won't turn lazy" and then she randomly came with this stupid reply "seriously stop acting like a doctor and like I'm resisting I never resisted on taking those I already gave up on my opinion about them making the body lazy jesus" she's just so mad about.. nothing. I mean, sorry that I'm being worried? And the way she communicated it seemed like she didn't take them so I just wanted to be sure and tell her that they are important and that they can help, I don't think I was being pushy since it was the second time I told her about them. 


She always replies like this on... about everything so we have atleast 2-3 fights every week, well atleast we used to because I'm starting to really just ignore her when she says something like that and later on we just talk about something else and act like it never happened. I don't want to start those pointless fights anymore, I'm done with it. She could have just said something like oh I haven't told you but I am taking them so don't worry. But instead she responds so aggressive and attacks me for really.. nothing. And I'm not a secure person so it's making me feel so unsecure, it's like I always do everything wrong. 

Could this be something regarding her ADHD or is this something else? And if it's ADHD, how can I best deal with this? 

Thanks for reading!