advice needed!

I had an assisted M/C on 22nd April due to baby stopping growing and did a test on Saturday (13/05) to confirm hcg had dropped to 0. The test came back negative however I have done a test today (after having mood swings and hot flushes) only to find the test come back positive. I did a further 2 tests of different brands to confirm this and they all say pregnant. Has this happened to anyone else? How do I calculate how far along it is? Thanks!