You have been feeling weak and dont know What is causing it?

Answer 1

Have your sleeping habits changed? have you recently stopped drinking caffienated drinks? Do you have other symptoms of sickness? Do you have heavy periods? Do you eat meat?

The answer to your question depends on the answers to the above. You may have something like a cold or even mononucleiosis. If you do not feel well, or your fatigue is interrupting your daily life, you should see your doctor.