Might be blood level is low. I mean haemoglobin level. Check karwa lain.
Sleeplessness.
Some stress.
Kamzori
May be before you did excercise a lot.
Leave you excercise for few days.
Have some rest. Khuwah yoonhi bistar par laiti hi rahain agar saain naheen to.
Kisi baat pay mayoosi ka shikar honay ki wajah say bhi aisa hota hai.
Koi sad news say bhi.
Multivitamin is a good suggtion someone did. I will say too. I myself parhaiz from dawai. lol