If you suspect implantation, test. Typically implantation bleeding is very light and does not last more than a day or two (there are some exceptions, however). If your cycles are 28 days, then if that was a miscarriage it would have put you at 4 weeks, but that typically isn't far enough into a pregnancy to see a clot of noticeable size (since the embryo is around the size of a grain of rice, and the yolk sack is maybe the size of a blueberry). Do you have a history of cysts or fibroids? It sounds like it could be a weird period, but with the nausea it could have been implantation or you could be getting sick.
Wow just 2 days ago the same type thing happened to me. There was just a smudge of brownish pink on the tp and this morning I tested and it was a BFP. It would be so neat if you were preggers too! Then both of ours would be about the same age lol. Test and let me know. I had 2 negatives. Once a week ago and then a few days before that. I had to buy an expensive name brand test to get the positive. :)
Oh and I had a miscarriage after 3 months. It was very small. And it was like when I was 3 weeks and I just had an extremely heavy period type thing. I saturated 1-2 tampons/pads an hour. Check with your practitioner.
it does sound like a miscarriage to me since you said you passed a bigger tissue like clot. call your doctor and tell him about it, he might wants to see you as soon as possible.