Gum hurts for 3 days while brushing.

Lower left molar teeth gum hurts while brushing. It also hurts if I touch the gum with my finger and rub on it. It gives sore and tenderness. . I had cavity filling done on first lower left last year. Nothing done on second left lower. What does it indicate if tooth gum hurts? One thing I done recently is, I am chewing food only on this side for last 8 months. I have lower jaw and neck pain on the right side and did some rct and wisdom tooth extraction that side. So I am using only the left side for chewing. Will that cause any gum pain? Does it indicate any severe thing? Scared about this. Already have right side pain going on and still no diagnosis or solution for that yet. Different opinions for that side.  Don't know what to do. Now this left side started. What helps for this gun and sore pain?