Welcome to Sclero Forums! I'm sorry you have Sjogren's and concerns about other possible illness as well.
Please keep in mind that I have no medical training at all, although I am studying for my Internet Medical Degree because I was told that my IMD plus five dollars will get me an extra fancy cup of java at my favorite coffee shop.
As I understand it, Sjogren's occurs by itself about half the time (Primary Sjogren's) and secondary to another connective tissue disease, like rheumatoid arthritis or lupus or scleroderma, about half the time (Secondary Sjogren's). So there is about a 50/50 chance that something else is also going on. However, that is pretty tricky to figure out because Sjogren's is a systemic disease, meaning it can also affect any/every part of the body, like the other CTDs. See Sjogren's Symptoms.
In fact, it can overlap with nearly all the symptoms of scleroderma, even including Raynaud's, difficulty swallowing, and heartburn. This makes it very difficult to sort out "additional" disease, and that is also why it is a great idea to consult a scleroderma expert if there is a question of scleroderma occurring along with Sjogren's; about 20% of people with systemic sclerorderma also have Sjogren's.
Yes, it is possible to have scleroderma and to be sero-negative for it, but there are specific symptoms required for a scleroderma diagnosis, so things like tight skin and pulmonary fibrosis would go a long ways toward the cause, but other typical symptoms of scleroderma (such as Raynaud's, difficulty swallowing, heartburn) would be discounted or ignored entirely due to the Sjogren's.
And until you see an expert for evaluation, also keep an open mind that the additional illness might be a seldom-seen complication of Sjogren's, or any other connective tissue or autoimmune disease. Most of us find that it is stressful to wonder about possible forthcoming diagnosis, so we recommend getting all the support you can muster, plus putting an emphasis on relaxation and stress reduction -- more hobbies, more fun, more rest too -- during this phase.