what is implantation bleeding?

Sophi
Warning: this post contains talk of periods and cervical mucus, if you find that TMI please don't read on x
So since i started my period I've never had it come early, sometimes a few days late but never early. So last month I had 3 days of light bleeding and mild cramping a week before my period was due. About a week after that I had heavy CM that was like clear and watery, and its continued and I'm still getting it. I'm having cramping on one side (the left). I don't know if this could be a sign that I'm pregnant, my fiancé isn't convinced that these are symptoms but something just feels different. We've been trying for about a year. Any advice would be much appreciated 💖