Could I be pregnant?

Earlier this year - February- I had the contraceptive injection. I hated the side effects so never went back for another round of the shots 3 months later. I haven't used any contraception since then. My periods have come back but have been irregular since then. In the last few weeks I have started having sex again and my partner has finished inside me quite a few times....I've started to feel all types of weird such as nausea, headaches, spotting, and serious fatigue.... I thought with the injection it could take a long while before fertility came back, I don't know a great deal about contraception and everything so I'm very confused...does having a period mean I am fertile again?...could I be pregnant?!