STIs - an introduction
It is not enough to know that your newfound boyfriend or girlfriend only slept with x many people - or even to know who they were. Sexually Transmitted Infections don't discriminate. Remember, when you have sex you're actually sleeping not just with the whole sexual history of each person you sleep with, but the sexual history of each person they've slept with. And so on.
If you think you have caught an STI
Don't panic. Some STIs can be dealt with by a standard prescription of antibiotics. Do see a doctor.
The feelings, though, especially if you have been careful with condoms and have still caught something, can damage your confidence and create feelings of self-disgust. They can make it difficult to trust people as well if it was the case that the person you caught it from lied to you. This not to say they have cheated - they could have had the STI for some time - or that they knew they had it; they may not.
If you do have a reasonable amount of casual sex then it should be unsurprising if you catch something at some point. The only 100% safe sex is no sex - though condoms do help enormously to hinder the spread of STIs.
There are lots of walk-in clinics where you can go for tests. If you go to your doctor first he or she will be able to refer you to the nearest one or have information about where to go. These places will not criticise you or frown on you but will offer you advice and give you the necessary tests and drugs to try to sort things out, if this is possible. Just make sure you are completely honest with them so they can be sure to give you the appropriate tests and make the most accurate diagnosis. They are completely confidential.
If your boyfriend or girlfriend falls silent, something's up.
Explain that you want to be 'let in' - then it can be good to give them space, tell them to call and let them come back to you