Gf Late Period? Pregnant?

Ok I have a question for everyone, im a little stressed over this but here it goes. Ok so my gf and I have been havin sex using condoms...She last had her period on feb 23 and we have had sex 3 times since then all using condoms...We have also played around like oral sex, hand jobs, etc. But never unprotected sex...Well this past sunday she feel like she was havin pms and she only had some brown spotting on monday...And then nothing she felt fine no cramps no usual back pain all durin the week, and then on thursday she said she felt like she was starting her period for real and she had very few drops of blood and she used a tampon. Then today she has had no blood at all...But has had a tampon in...Im worried because idk whats goin on...Is this her period and its just light or could she be preg? Could it be that the tampon is just delayin it kinda cuz she says she doesnt regularly use tampons until she has more blood flow..Id like to hear ur opinion and if anything like this has happened to anyone before...