Welcome to A2A, cyberdood.
It may well just be the novelty of the situation, and get better with experience.
Many of the pills on the market are quite ineffective and sometimes dangerous. The most effective drugs for premature ejaculation are the SSRI antidepressants, but I think it is too early to consider them. Another type of treatment you might find in the high street is the 'last longer' lotion - this contains a local anaesthetic and essentially works by making the experience less enjoyable. I think it's not a good idea because it trains your penis to be used to less sensation, whereas we want the opposite.
If the problem persists, I think the first thing to try is the 'stop-and-go' method. In this, when you are getting close to the edge but not yet at the point of no return, you stop all stimulation. Then, after a bit, you start again, and repeat the process as often as you like. The idea is to train yourself to last longer, so you can use it in private masturbation as well as with your girlfriend.