MISCARRIAGE?

I’m 18 years old, im a full time student and part time worker. I’m not looking to have a baby at this moment in time. But anyway - few weeks ago I’d taken a test which came back as a faint positive and the day after I did another which came up negative. I’m on the pill called cerelle, you take it everyday with no break. I don’t really get a period, every so often though I do get some black/ brown tissue like discharge which I’m guessing is old blood. I’ve been getting cramps and things. So I took a test as I don’t really get a period so I wouldn’t know if I was pregnant if I was someone to get pregnant on birth control. So last night I was in the shower and this came out and today I still have stringy stuff coming out. My sister has experienced a miscarriage so I asked her for help to see if she could tell me what it was. She thought I’d miscarried. Today I went to the doctors to get checked as she recommended. The doctor made me take a test and said if I start being sick then I’m to come back and they might do a scan to see if there is anything still inside me. Although she didnt tell me if it was a miscarriage I had. Have any of you experienced this? Or could tell me what it could be And if it is a miscarriage?