My grandma had a stroke nearly 4 years ago & now has dementia. we haven't visited her in a month & now she constantly chews on nothing. why is that?

You said it: You said it yourself - she has dementia. (it's also possible that she is being treated with medications to help her stay calm which can cause tardive dyskinesia: a constant chewing motion of the gums and lips).

Difficult to say: This sounds like a movement disorder called tardive dyskinesia (typically repetitive, chewing mouth and jaw movements), which is usually a side effect of certain medications. However, there are other conditions which can look similar, such as an essential tremor, parkinson's disease, or even poorly fitting dentures. The best way to understand her symptoms is through medical evaluation.