Mouth Ulcer since 8 months. Knee pain, &

It's been almost 8 months my mother is suffering from mouth ulcers back to back. Back story- She was on major diet 1 year back left all grains & rice & lost pretty much weight due to her knee pain, & thankfully knee pain recovered but since then she is complaining about constipation as due to having protein diet, from then she started having all the laxative like isabgol & few more to relieve constipation & from then only she started complaining about mouth ulcers, treatment was on at that time like antibiotics and all , get recovered & comes back again after few days. Due to antibiotics constipation comes again & repeat condition. How to treat this. Big ulcers with pus. Given blood report to check vitamin deficiencies. What can be the possible cause ?