You had unprotected sex with two partners who are familiar? to you within 48 hrs. After that you were masturbating aggressively. Now you have pain like pulling, a tearing sensation of the skin. Later you noticed a painful red bump. After 2 days it has become little worse, become bigger with a centre dark spot. Your doubt, whether it is due to masturbation
Before coming to a conclusion you must understand certain facts.
1) Aggressive masturbation can produce symptoms like this
2) STD - Too early to produce symptoms like this, but can't be ruled out
3) HIV - Unlike a lot of other sexually transmitted diseases, HIV does not produce symptoms on the genital areas. In fact, part of the reason the disease is so widespread is that it takes a long time to produce any symptoms at all. HIV destroys the body's ability to fight other illnesses, and it can take as many as eight or nine years before an infected person develops serious symptoms. For this reason, regular testing is key to detecting the disease and preventing new infections.
Let us hope that it due to your aggressive Masturbation - which will heal with antibiotic + anti inflammatory treatment along with obstination from sex and masturbation till you get alright.
If it due to STD you need blood test
2 or 3 times within 1 month,
it is recommended HIV testing after a potential exposure at 6 weeks, 3 months, and 6 months after exposure.
Any how consult a venereologist and follow his advise.
I wish the cause should be due to masturbation. Every time luck will not help you. Don't take risk of your life. Always keep in your mind that "PREVENTION IS BETTER THAN CURE"
Best wishes and best of luck