I was pregnant 2 weeks after I had the Mirena taken out, so I know its possible to fall pregnant virtually straight away. I miscarried my baby, but fell pregnant again 6 months after. I have seen other stories of women who have fallen pregnant straight away and went on to have a healthy pregnancy, however, in saying this there are women out there who have heaps of trouble falling pregnant after Mirena. All women are different. I wish you all the best.
Unless you are trying to get pregnant, you should always use protection. But to answer your question, if you do not get your period in another week or so, and should have, then it's time to call your Dr. On the other hand, when changing or stopping ANY birth control (hormones) your body can take up to several months to regulate itself. Don't panic (or get too excited). Your best bet is to wait a little longer, and be mindful of any other symptoms. Call your doctor in another week or so if you still haven't gotten your period.
Any time you have sex, you risk the chance of pregnancy, so yes, you can be pregnant. There is no safe time to have unprotected sex, as ovulation can happen a lot sooner or later than expected. If you don't want to be pregnant, you need to always use protection from start to finish. Are you trying to get pregnant?