Hi Watermelon,
Unfortunately, it's all a guess without exact dates. Even then, there are variables that we can't control. Anytime you have sex with multiple partners during a cycle there is a chance that any of them could be the father.
You had a NORMAL period at the very beginning of June?
You had sex around the middle of the month. You normally would ovulate between days 11 and 16 of your cycle. The egg is viable for about 48 hours, sperm for up to about 5 days.
You could have gotten pregnant having sex from about the 6th to the 20th - assuming June 1 was when your period started.
It doesn't matter how long it lasted. There is ALWAYS a chance of pregnancy with ANY sexual contact. Precum can contain sperm and most guys can't tell when that's happening.
Based on your post it is "probably" your boyfriend at the beginning of June. You will need a DNA test to confirm paternity. It can be done now using your blood. Google, non invasive DNA testing.
Good luck.