Most people will find the first few weeks at uni exhausting - making new firends, going new places, learning new things all need more mental effort than when you were at home/school. For instance when I'm at home I can walk to town on "automatic pilot" - don't need to concentrate at all. But when I came here last year everywhere I went I didn't know whether it was left or right! It's like adrenalin pumping 24hours a day.
And at the same time there's alcohol, parties, late nights...and no Sunday lunch on the table I could eat in my pjs having been up for 5mins
All combined means exhaustion. Once you get used the new environment it'll get better...it did for me.