Also, take a look to where you are playing inside the actual fret itself. The true tone of the note resides towards the bottom of the fret so be mindful of that and that could be your issue. Like Christopher said, you could also be slightly bending the strings which is very common for beginners to do as you are still developing the proper arch of the finger and attack on the string. Don't worry too much. This is very common and you will work though it.
:o