Uy, mama, please go to your OB or go to the nearest hospital right away to get checked, spotting in the third trimester needs to be looked at by a doctor, kaya don't wait. Better safe than sorry when you're this far along na, talaga.
I know it's scary, but spotting can mean different things and only your doctor can tell you what's going on. A few of my mum friends had similar situations and everything turned out okay, pero they went in immediately para sure. Fingers crossed for you, let me know how it goes!