Well, it's normal for it to feel awkward at first. I mean, with me, the first time my beau and I played was by taking my temp rectally. Yes, medfet turned me on but I was freaked out by having a thermometer in my bum. Let me also add, Luis, that I'm a nurse and I know what I'm doing and taking an RT isn't rocket science and can be done by anyone. The point is, you're exploring something new here, so it's normal to have doubts, be nervous and even get freaked out. More so if you don't know what gets you off. That's why you should talk with your girlfriend, find out what gets her off and turns her off. Then try to incorporate what she likes to play, then avoid what turns her off.
For example, if she's anal erotic: you can take a rectal temp or do a rectal exam. I'm sure her clit is sensitive, lol, I haven't found a woman who isn't. So, you could "test for clit response" then rub her till she comes. Be creative, do your thing, don't give up just yet.
What's important is communication and connection. You need to talk to her and find out if she sees medfet as a turn off, if she does then you'll have to settle with vanilla sex, it might not be catering to your medfet needs but it's still sex, lol.
As with your assumption that her discomfort is a sign that she didn't like it, you can't be possibly correct about that. I mean, I was uncomfortable the first time I had a rectal temp taken but the sensation felt good. I didn't recognize that I enjoyed until the third time. And again, that's because it's a new thing. But now it has become a favorite and the beau makes sure to take my temp regularly. lol
So, Luis, don't lose hope, just yet, talk to her, find out what makes her tick and do your best. Then tell us all about it. Good luck!