Hi,
Good questions. Friction can irritate the skin on the penis. If your technique is causing friction, you either need lubricant or a change in technique.
Your girlfriend is not responsible for your health. Only YOU are. Make an appointment and get tested. Check the laws in your state, but most places you can see a doctor without a patent by the age of 16. You are also at the age where they do not need to be in the exam room with you if a parent must go. Just be polite when your name is called and turn to your parent and say, "This should not take long." Then give a loving smile and walk away. If you need a reason for the appointment, just tell them you have a rash you want seen.
Tell your doctor everything. Masturbation and girlfriend. If he does not have the full picture, he cannot order the correct tests. This is also a good time to let the doctor know to call you with *these* test results. Give him your cell phone number and ask him to put a note in your file to call only you.
Now, if anything comes back positive, you will need to have that talk with your parents. Then you will need to talk to your girlfriend, because one of you has not been telling the truth. When you talk to her, be respectful. How or who is none of your business. Be supportive of her getting treatment, because you will both be going through the same treatment. If it was you not being honest with her, this us when you man up and take it on the chin.
Hopefully, that last paragraph is just advice you pass on to a buddy one day and has nothing to do with you. :)