vaginapagina | Abnormal brown bleeding mid-cycle

Background: I have been menstruating since the age of 10. I am now 27. During that time, I have always been extremely regular. My cycles are between 28 and 30 days, with ovulation occurring around the 14th. I can always tell when I'm ovulating. I've only ever been late a couple times in my life, but the next month my cycle is back to normal. I've never been pregnant and I've never been on hormonal birth control.

Problem: A day or two before my period last month, July 18, I experienced cervical mucus tinged with blood. Prior to that I had a lot of clear discharge like once might have before cervical mucus. I've never had cervical mucus around my period, only ovulation. My cycle was around 6 days, somewhat long, and the entire time I never had bright blood, only brownish and never too heavy.

Just a few minutes ago I went to urinate and found that I am experiencing moderately heavy brown spotting. This is roughly day 15 of my cycle and I should be ovulating, but I haven't had any signs of cervical mucus yet. In fact I haven't really had any discharge since my period ended. And now that I think of it, my husband and I had sex on the 30th and I had spotting afterwards (although not as extreme as right now). I've never had post-sex spotting before. I took a cheap pregnancy test on Monday, but it was negative.

Other symptoms: For about three weeks, I've had tender breasts. I feel hormonal, on and off. I lift weights 2-3 times a week and walk 3-4 times a week, but I don't exercise excessively. I'm 5'5 and around 125-130 pounds. I've been trying to exercise off the last 5-10 pounds, but it's been difficult for me to lose weight the past two months. I'm on anxiety medication and do not feel particularly stressed.

I'll be making an appointment with Planned Parenthood for next week, but I feel a bit panicked right now as I've always been regular as clockwork. Should I take another pregnancy test? What could this be? Thanks in advance.