If it is particularly large and sore/unusual for you, then I would either go see my doctor or go see my dentist. I'd almost be more inclined to see the dentist than the doctor, but you may have to see both or at least contact your rheumy or GP.
Yes - many gum issues with Cellcept - although overall its been a huge help w/SLE. The dentist is the best help. My gums actually "split" open under the gum line as if it had been cut (painful & swollen). I had a large "gash" that wasn't closing. The dentist got me in within an hour of calling them & looked at it. He gave me a perscription blue rinse w/antibiotics inside it. But he had me apply it directly to the gum "gash" 3x daily for 2 weeks w/a long Q-tip. (Otherwise it would of turned any plaque on my teeth blue if I had rinsed w/it). It worked. I've had this occur multiple times with no real reason - no ones sure why but I do this each time & it closes & goes away & you cannot tell it was ever there. (The Dentist thinks perhaps my gums may become swollen just as my body does at times & split open from the stress. I floss daily & brush multiple times daily w/my Sonic-care too & it still happens!). Although honestly it really freaks my Dentist out as he's worried about working on my teeth & causing me any issues.
He also recommended "bio-rinse" which you can get at any drugstore to rinse w/during the day & at night. It gets rid of any bacteria in your mouth & adds healthy items back in to keep things "healthy".
I have to say my heart sank when I read your post. My mouth problems have worsened considerably and I was hoping starting Cellcept would help me.
There is one part of my gum that is very tender but doesn't look inflammed. My 'magic mouthwash' isn't cutting it anymore so I use the steroid paste everyday and also hyrdrocortisone pellets. I know I need to see a dentist soon and you've helped me with the decision to make the appointment.
I hope you go to see your dentist and get some relief soon.