L.C.
K.,
You didn't say when you took the first pregnancy test at the clinic. I am assuming that it was last month and perhaps you took it too early. If you took 3 pregnancy tests and they were all positive then you need to go back to the clinic and have another one taken there. Let them know you are concerned about the large clot. They will be able to help you medically if you are pregnant or not or if you did possibly have a miscarriage.
Perhaps you are just having a regular period and it was just late. Stress, change in weight, more frequent exersize, etc... can all lead to an inconsistant cycle.
You are wise to follow your instincts and go to the clinic today. Let us know what happened.