hi guys, Kayden hasnt been putting as much weig...

hi guys, Kayden hasnt been putting as much weight on lately, he is 4mnth 3wks and weighs 7.5kg he has only put on 20grams in 2 weeks. He has been teething because he got his 2nd tooth on 2days ago. While this has been happening he has woken up 1-2 times a night to have the dummy, and normally goes back to sleep. But just tonight i thought hmmm, i might give him some formula also and see if he will sleep through the night?! before i fed him it didnt feel like i had much milk, like my boobs wernt FULL. i gave formula to him after he had a feed, and he took anoter 100mls! I dont know when or how to tell if i dont have enough milk, and maybe he isnt sleeping through the night any more because he isnt getting enough milk?! What do you think? or why is this happening? do you have any suggestions? or has this happend to you before?