Bleeding in between my period.does this mean that I am pregnant or have a serious disease?!?

Bleeding in between my period.does this mean that I am pregnant or have a serious disease?!?-now here is the thing-i had my period about 2 weeks ago .i am not on the pill or anything,just using condoms.i had s3x with my boyfriend on the first day of my period(without a condom) and have been having sex regularly for these past 2 weeks(using condom). this past period was very light and short in comparison with my periods in the past. now yesterday i realized that i am slightly bleeding/spotting ,but this has never happened to me before and the spotting is really weird with some unusual tissue which is being discharged,the color is very dark ,would say something like dark blueish /brownish . i am so worried that this can be a sign that i am pregnant ,because my breasts have become very sensitive (the nipples) as well.could you advice me something or tell maybe you had something like this happen to you.by the way ,i am 21. every answer is highly appreciated ! thank you !

Best Answer:  The bleeding between periods can be an indication of a hormone imbalance such as with the thyroid (an example only but something I've had happen to me before).

I would recommend seeing your doctor for further evaluation of the situation to see what they think could be going on.