Losing Control?

So i've been fucking this chick for about 2 months, been having a great time. The only thing is that lately, I've been having some trouble lasting as long I used to. Up until a couple days ago, we would normally fuck for at least an hour and I would have no problem lasting (condom or no condom). But all of the sudden in the last day or 2, shit just feels too good! Like damn, I gotta take it a lot slower and have to try to distract myself so I don't come, and I still can only last maybe 30 minutes. This obviously doesn't ruin sex for us but I would waaaaay rather have it be like before where I really had good control and could fuck like an animal for hours. 
 
So I have a few questions about this:
1. Am I just in a slump? Is this just a stupid quick little phase that will pass?
2. Should I go back to using a condom for a little while to, like, recondition myself or something?
 
any other advice or general knowledge is greatly appreciated!

 

 
ah that's how i've been pretty much all my life (i'm 20), but it's like something drastically changed in the last couple days. it used to be reaaaally hard to finish during sex, like I had to really concentrate in order to come; now, I probably could be a 2 pump chump if I wasn't concentrating on not coming. jacking off can maybe buy me a few more minutes, but there's a huge divide between fucking for 2+ hours and fucking for 30 minutes that jacking off can't solve-it's the root of the problem that I want to figure out. 
 
argh, this is just sooo frustrating, my main problem is that it's getting in the way of me pleasing my lady..
 
Has anybody else ever had a change like this?
Does it happen when you're exclusively having sex with one person after a while? Am I losing my attraction to her?