I think it unlikely that you are pregnant, but it is always best to rule it out. I understand that buying a pregnancy test at your age would not be easy, but the school nurse may be able to help.
There are many reasons why you might bleed between periods. You say you had sex with your boyfriend in July, have you had sex since your last period?
I'm thinking that the bleeding may be directly linked to sexual intercourse which can happen when you are new to sex.
Otherwise it could be down to contraception, you don't mention that you used any (I hope you did) but if you are on the pill, have a patch or similar, the hormones in these could be affecting your periods.
It really is best to see a doctor if at all possible, as there could be so many causes for this. Most causes are harmless, but as you are obviously worried, I think you need to be 100% sure that all is well.
Please try not to worry though, as this will only make things worse: stress can be another cause of missed or heavy periods - we girls just can't win!