Hi can you help? I'm 21 and have unprotected sex this month and last month can't tell you exact dates. Anyway I was due on my period and I have had black like tar on my pad and about 2 days I had a bit of bright red blood when I used the toilet and wiped(sorry if tmi) I had nothing on my pad for few days now and when I wipe it's now brown. This has been going for 4-5 days. Usually my cycle is a heavy flow 3rd day and clots and cramps I've had no cramps or clots and also I've had diarrhea for 3 weeks which I'm getting blood tests for tomorrow, please could you help as I'm worried?

I'm sorry but it could be so many different things we can't speculate online. When you have the blood tests, are you also going to be examined and have a chance to talk to the doctor, or are you just going to the lab?
Different colors of blood/tissue are normal and indicate for the most part how 'old' it is, how much exposure to air it's had.
We can't possibly know if the diarrhea is related or not.
As in all cases related to questions about pregnancy in the first month, the answers are always:
You might be pregnant even with blood flow
You might not be pregnant even with no or unusual flow
You might have miscarried

And of course always chances of STD, even with partners you know, so if you are being tested , may as well double check for that also.