My girlfriend and I have been having sex for some time now and I have been having sex with only her, I also believe she has been having sex with only me. I was a virgin when we started and she wasn't. She says she had sex with some guy about a year before I came.
Starting about 3 weeks ago, I started to feel a bit of some stinging pain on the foreskin of my penis whenever I washed it. I ignored the problem and kept up my hygiene, washing thoroughly, daily and the pain disappeared in a week.
The big problem now is, this past Friday, I had sex with my GF without any problems. Going onto the second round that night, I couldn't penetrate,so we just had it with only the head in. Afterward I realized that the Glans was swollen and as well as the skin and it was painful. The next morning I examined myself and found that I had what seemed like a reddish sore on the part of the foreskin that is attacked to the glans(the part that has small 'cord' if you get what I mean). There where also two whitish, sores on the foreskin.
I started to use Betadine in an effort to prevent an infection, and I still am using it.
Yesterday, the skin covering the both the reddish and the whitish sores ruptured whilst I was washing up the area, I don't know if anything came out of these sores. I keep calling them sores because they were definitely not blisters.
Today I examined the area and noticed that small patches of a tiny rash are appearing there on the inside of the foreskin, and it's a bit itchy. There is no discharge coming from my penis.
Can someone please help me, Id like to know what this could be, if its an STI, what are the possibilities and if not what could it be...

First of all stop using anything to clean except warm water, next go to the doctor and get checked out, it's the only way you'll find out for sure, if you have an STD then ask you doc how long this would have took to occur, if it's less than the time you have been with your girlfriend then it's time to end things, if not then you both need to go to the doctor to clear up the STD.

Okay I agree with you that I must see a doctor but is it possible. But does anyone know what it could be if not an STI. Or the most probable STI.

If you are using soap and other detergents to wash your penis with then this could cause you to get a skin irritation.

From what you have described you probably have herpes.

If you have herpes there is no cure for this STD. The only thing you can do is use Vitamin E squeezed from a vitamin e gel cap onto the sore to help it heal and to stop it from hurting. Someone got herpes and gave it to you. You obviously were not using a condom otherwise you would not have this problem now. In the future use a condom so as to not infect another female while having sex. Also, be sure not to have any female give you fellatio while you are having an outbreak of herpes or she could get herpes in her mouth. Yes, this can and does happen to females.

Also check out photos online of herpes on the penis. Bet your penis looks the same as the photos.

The doctor has given me a shot of benzamine penicillin. Now I can't walk properly, its so painful. Since I have been given an antibiotic, does this mean it could be some bacterial infection like syphilis? How bad could it be?

Contact your doctor and inform him it hurts terribly and you can't walk. Don't delay in telling him your new symptoms. He might have given you something and you've had a reaction to it.

Oh, I can walk now, it's the injection. Its two large doses on both buttocks, he warned me of the effects like the pain. Now just waiting to c what happens in the coming week... Thanks 4 your help and concern...