When to worry about bleeding during pregnancy?!

So im about 3-4 weeks pregnant. I took a pregnancy test sunday of last week and i was pregnant! Well fast fwd to 2 days ago.. i started spotting and it would stop and start again and it stretched into yesterday, well today im just spotting brown and it has like brown discharge from what i think it is.. I went to the hospital and my hcg was at 75... im just scared cause this is my second kid but my first i didnt know i was pregnant til i was 12 weeks so i have no idea what to look for. I havent had cramps and i never got a heavy blood flow i didnt even see blood red.. only pink spotting when i was bleeding. Can someone pleaseeee help!