Possible miscarriage

I went to the ER yesterday because of cramping and bleeding. They couldn't see anything at all on the sonogram but my tests came back positive for pregnancy still. Also my HCG level was in the 300's, I'm supposed to be 6 weeks so that's a super low number. Once we got home I passed a finger long piece of tissue, I don't think it was the baby just because it was a solid size. The OB is supposed to call me and set up an appointment to get my levels checked again.  Getting pregnant is already hard, keeping the baby inside so it can develop is harder. If it is a miscarriage then this will be my 2nd in 6 months.