Bleeding since Mirena fitted

Hey girls, I've been on the cilest pill for about 5 years and it worked really well for me, I always knew when my period would start and stop and never spotted and never got pregnant! I developed a condition which means my pills may not be absorbed so was advised to change to another contraceptive. I had the mirena IUD fitted about 3 weeks ago, which I'm fairly sure brought on a thrush infection a week or so after. Also I have been having to wear STs cause I keep randomly bleeding a bit... I'm guessing this is a kind of spotting... But I've never had this problem before so I don't know how to deal with it... How do people know when they can have sex if they never know when they're going to find blood?! I was told I probably wouldn't have periods at all on the IUD ... Also do people get lots of infections with their mirena coil? I seem to be susceptible to them 😔 thank you!!