Is my Kitten in heat or having anal gland problems??

Hi, I am new to this site and a new cat owner. I go my kitten, Snickers, when she was just a baby. The girl I got her from said she was 6 weeks but the vet says she is thinking closer to 4-5 weeks. She is now 3 months old, almost 4 months. She is still a baby. Is it possible for her to go into heat this young? She has been acting funny lately. My boyfriend said she stuck her butt in the air with her tail to the side yesterday, which is a huge hint that she is in heat. She hasn't done it again since though, just the one time. I was cleaning her litter box this morning and she hoped it while the lid was off. I noticed that she had let out a yellowish discharge then moved to a different spot to pee, very little pee came out. My question is did she discharge becasue she was in heat? Or do her anal glands need to be expressed. It was not a brown discharge, it was runny but a thicker consistency then pee. She has also been licking herself down there quite a bit. 

She gets fixed next Friday, but I am going to call the vet today and double check but wanted to ease my mind in the mean time. 

It is possible for a 4 month old kitten to go into heat, unusual but possible.  The discharge you are talking about is concerning.  I don't know much at all about kitties in heat, I was fortunate that my little girl kitty wasn't one that went into heat very early.  I think it is good that you are checking with your vet, my first thought is possibly some type of infection.