Bedroom Issues?

I'm a guy in my mid 20's and have never had any issue getting an erection. However, when I was with my girlfriend a couple weeks back she was going to go down on me (this was the first time we had done that as we've only been dating for a short time, roughly a month or so). The night before I couldn't keep it down. When she started to go down on me I wasn't getting an erection even after several minutes. Embarrassing, needless to say. Yet the previous times we hung out and were just laying there cuddling/spooning I couldn't keep it down. I used to be able to get erect without issue. I'd even be just sitting at work and be like, well I guess I have an erection. This incident occurred three weeks ago and haven't been getting erections nearly as easily if at all as I did in the past. Then two weeks ago I was able to get an erection for awhile while she was going down on me, however, I started to get flaccid but she wanted to watch me do it myself so I ended up ejaculating. However, since that time I haven't really gotten erections, nor have I ejaculated. Has anyone else run into this issue and what solutions are there? Like a month ago I had no issue. I've noticed I have a way lower sex drive now as well in comparison to a month ago. Is it is possible since I've laid off on fitness and weight lifting over the past year my testosterone levels decreased causing the issue? What's going on? I don't want to go to the doctor for this as I'm still on my parents medical for now.