Hi I'm 18 and I have this problem. I had unprotected sex a month an a half ago and I missed my period. But I have bled lightly but redish brownish and very little maybe up to three times each day,this is the second time it happens and it lastes up to four to five days. I have taken three home pregnancy test but they came out negative. I'm still worried cause I haven't gotten my period, and I don't know what it is.

You can go and get a blood test from a doctor or at the hospital and that will tell you for sure.
Sometimes periods can change with age, stress or any number of other factors. Sometimes they can be rushing and bright red other times they can last 1-2 days and be brown discharge with blood spots.
The best thing would be to get a doctors test and an exam in case of any unexpected STD's and maybe to get a pap smear, if you have not yet had one or not had one in a while. Sexual health is so so so important, ignoring it doesn't make anything better. As embarrassing as it is, you've got to look after yourself, so get get checked for pregnancy and anything else that may be there.
If money is a concern, you need to ask for a small loan from your parents, friends, distant relative or even credit card it if you don't have insurance. Your health and peace of mind is worth it.

A lot of things can affect your period (as in making it late), a big thing is stress, exspecially when its about being pregnant.
The reddish brown stuff could be spotting, then again it could just be you're having a light period.
HPT's aren't always acurate, you should go to a doctor and have him give you a pregnancy test

If you had unprotected sex and your period is now irregular I would set an appt with your doctor for a blood pregnancy test to ease your mind.

But periods are not like clockwork... they can vary woman to woman and cycle to cycle. It is not uncommon to be late, have a change in flow or length, or miss a period altogether once in awhile.

If you bled for 4-5 days, regardless of the flow... I would say you just had a light period.

I don't have periods unless they are medically induced... even then I just spot for a few days... use a pantyliner at most. But those are normal for me.

But if you are concerned, consult your doctor.