Sounds like a virus or infection. If you can take paracetamol that'd be ideal to reduce any temperature you may have. If you've not already, it might be a good idea to take some berocca which helps me if I'm feeling run down. Other than that maybe some lucozade which has electrolytes and stuff in it that could help you too.

If you're not feeling better soon it would be ideal to go to see a doctor, maybe at a walk in centre or polyclinic if you have any in your area which would mean you wouldn't have to see your GP.

I hope you feel better soon.