I really need health advise?

OK. this is really long but i need some advise. Im 13 and i had this ‘stomach bug’ where it gave me stomach craps, diarrhea, and stomach gurgles that made me wnt to go to the toilet. so i had this for a good week, then i would just have a queasy stomach the i would have to pass gas (fart) so i had that for a week and also having diarrhea i lost some weight.
So then when it cleared up i went back to school for about 2 weeks, then this whole thing started up again. the same thing and ot lasted about a week then went for a few days and came back again for only about 4 days.
so then i went to the doctors for a few times during this and they said its just a stomach bug and then they wanted a blood test. so then that came back with showing me to be anemic. and since i have lost allot of weight threw this, and im really skinny now and pale from being anemic.
Im also weak. like my heart pounds when i have just walked up the stairs! i feel faint when i walk outside and i get queasy when i walk to much. the thing is, this ‘bug’ comes back when i eat something. pizza to be prissiest. my parents looked into it and they think its lactose intolerance, i think it is too, ive gone off dairy and im normal. but my main thing is how to i not be weak anymore? im not going to school until im not weak. i hate it and i need some advise on how to build my strength up again. thanks if you read all the way there this, ha ha just im a little worried 🙂