1 month Baby spitting milk. Milk or not?

I m feeding my baby every 3 hours but I think his stomach is not getting full after my breast feeding...I tried to give formula milk but he spits up whenever I tried to give... Also from 2 days whenever I breastfeeding after that he spits up some milk like sticky liquid..is it normal?? Also doctor told me that after every feeding you have to burp for 20min which is now too hectic for me..and dur to this I having chest pain now. Pls suggest what to do?? 1. Spitting sticky liquid after breast feeding is normal or not? 2. He is having gases problem..he passes many gases . What to do? 3. After my breast feeding he wake up every 2 hr and demand more milk. I think his stomach is not getting full..and after that when I tried to give formula milk through spoon and katori he rejects every time..now pls help me in this case 4. Is it compulsory to burp 20min  everytime after feeding?? I m very stressed and very confused what to do...I don't know whether my baby getting enough milk or not?