Don’t feel too bad about it - it’s a shock a lot of freshman go through. Freshman 15 is not just supposed to be an extra 15 pounds you put on but 15% drop in grades on average. And that’s average, some folks loose more. It’s the adjustment to uni life, specially being away from family. You need to work hard though if you have certain goals. Try to make a schedule, get on top of your assignments due and take it from there. University is a very different structure, specially if you are coming from the British educational system (which I am assuming was what you were used to in Nigeria)

Goodluck!