Fainting due to low sugar level or more serious?

Yesterday I almost fainted because I rushed around for 20 mins speed walking then stopped and felt faint My eye sight went slightly black in parts I could only partly see I lost my hearing and felt dizzy and weak After some sugar and a sit down I felt fine Today I felt lightheaded and a bit foggy in my mind like feeling weak Also recently I had a thumping headache that wouldn't go away all day even with paracetamol- I'm not sure if that's related Is it just low sugar levels or something more serious like brain related or diabetes or something? Help I'm panicked !!! Ps I'm perfectly healthy and in good shape but could fit up and do more exercise! Thank you :)