Aiyoh, bleeding between periods boleh happen for so many reasons, hormonal changes, stress, infection, or even just your body doing weird things lah, especially if you just had your period! Since your UPT is negative, I'd still suggest checking with your doctor just to be safe, sebab darah pucat and lendir macam tu boleh indicate something needs a quick look-see, and better to rule things out than worry, kan? A few of us dalam our mum group pun pernah experience similar things and most turned out okay, tapi it's always good to get it checked properly so you know what's going on with your body.