It could be a number of things which is hard to say for sure over the internet.
A few things that it could be are.
Anemia. An iron deficiency.
Not eating healthy enough and not getting enough sleep at night.
Our bodies need the correct fuel to perform right. Just like a car if it doesn't get enough fuel it won't operate right.
Make sure you get enough sleep at night, eat enough healthy food, get enough exercise etc.
Take an all purpose vitamin for awhile in case you have a deficiency. Your problem could also be related to medication that you're taking, your mental state, or your lifestyle.
A doctor visit is recommended to get a true diagnosis.