Lots of reason for yellowish discharge. Some of my pregnancy books say that a change in your discharge is just pregnancy related. However, if you're experiencing other symptoms (burning, itching, redness, fever, etc) then it is most likely an infection.
Like spade22 said, call your doctor and he/she will take a swab and test it. Then you'll know if it's an infection and be told how to treat it safely!
Could very well be an infection amongst other things - your best bet is to call your doctor and get checked out in case you require treatment.