Methotrexate and Lamictal

I was just diagnosed with RA and was prescribed Methotrexate (three 2.5's once a week).. I am on other medications such as Lamictal for Bipolar II, Synthroid for hypothyroidism, and Zantac- for possible stomach ulcer.. I read under the Lamictal interactions, that I should notify my doc if I am taking methotrexate.. what interaction do they have? And also, I read under methotrexate that it can cause stomach ulcers, which I think I already have before even starting this med.. I'm just real nervous about Methotrexate and want to make sure I'm not mixing meds I shouldn't. My doc didn't say anything about it, but I assumed she knew based on my medical file!
Another question- I read that having thyroid disease can also cause other autoimmune diseases...could that have been the cause of me having RA?