Sometimes I go flaccid while having sex with my gf. Not too often.

Routine maybe?
Like you get bored and start thinking about other shit.

Maybe you're cumming too often.

if I have sex multiple times a day for a few days in a row, I start to lose energy and have to take a break. I'll just focus on my girlfriend by eating her out etc until I have energy to fuck again.

It happens sometimes, man. Especially if you're using condoms. You're not a machine, life isn't a porno, I don't think any guy actually has a perfect 100% success rate with this stuff. I wouldn't worry about it unless it becomes a more frequent/chronic problem. Stress/anxiety are big factors here, so you could easily make it worse by worrying too much about it. Just don't treat it like a big deal when it happens, use your fingers on her to keep her warmed up and try to get had again.

Good chance you need more cardio, or just have your girl on top doing the work so you're not getting winded