Yes, may be it is because you have eaten something not healthy or you have been around sick people, or you have put very few clothes on you on a windy night? We could tell a lot of different reasons for your sicknes now. But is any of these the right reason? May be not!
Have you thought about your attitude towards life, health, love and so on. Have you thought about the thing called negativism. Yes, this could cause very big troubles in your life, health, love, happiness, etc.
All the negative thoughts that we have, the stress, the nerves ... all of this come back to us with great power but not to help us, just to cause more problems.
So the one and only decision to feel and be healthy, in love, happy ... is to think positively. To be grateful for what you have and to believe in good things and positivism.
You have to believe that things are going to be fine so that they change towards good. You have to believe in love, so that you feel in love. You have to believe that you are healthy, so that you really be well again.
Just think positively and everything's gonna be fine again. ;)