Im scared it could be a std cause I did find out my boyfriend was cheating on me

I had my regular period June 30 it lasted for 4 day I had sex with my boyfriend The day after it ended he cam in me. On June 14th I started spotting blood it wasnt really noticable I only saw it when whiping myself nd it was a light brownish watery color. So I really didnt think it was blood just thought I wasnt whiping myself good enoug. I had sex on the 16th with my boyfriend nd after we had sex nd i was cleaning myself up I realized that it was blood cause it looked pink since Then I havent stopped bleeding. I dont think its my period cause its not heavy at all. But I have been wearing a pantie liner nd a lil bit of blood gets on there. I feel no cramping or any pains. When I told my mother she said it could be because of stress and The heat. I havent told her about the part when my boyfriend cam in me cause I dont want her to kno that I might be pregnan. I made a doctors apointment for next week. Idk im really worried im scared it could be a std cause I did find out my boyfriend was cheating on me Thats Why been so stressed so idk it could be stress or it could be pregnacy cause he did cum in me 2 weeks ago. Please help me i cant wait for a doctor I need to kno something now. Im 20 years old. Has anybody else been through anything like this