I Went On The Contraception Injection For The First Time

I went on the contraception injection for the first time in October. It were to last for two months until the next injection was required. However I did not go back to have my second injection in December. I had unprotected sex after that and am fully aware of how long this particular contraceptive takes to get out of your system and the chances of becoming pregnant straight away are very slim. But I was just wondering seeing as it was my first time that injection and it not being a very high dosage may mean otherwise. It has been roughly between 6/8 weeks since this incident and I am not feeling myself. For instance, I am feeling extremely fatigued and get tired in the middle of the day and literally struggle to keep my eyes open. I can also sleep for hours and hours on end and still be tired. My breasts are super sore and my nipples hurt by the touch. I have also been getting bad headaches very frequently. Except I have had vaginal bleeding a couple of weeks ago. However it wasn't like a normal period and the discharge was more in the form of dark brown blood clots and weren't very heavy but did last about a week. All of these symptoms have come up since then. So I would like to know if there might be the slightest chance that I may in fact be pregnant.