The tooth has three layers enamel dentin and pulp. When the decay reached pulp it provides a free access to the root of the tooth which inturn is surrounded by bone. Pain on pressure and chewing is a symptom of infection reaching the bone from the pulp canals.When an acute infection occurs there is swelling of the gums you may even notice a boil draining pus from the gums.Do not delay treatment you may need to do a root canal or extract the tooth depending on the existing tooth structure and extent of infection. Antibiotics may be needed prior to any treatment in case of swelling.

Hi in your condition infection has reached to the root .you need immediate root canal treatment followed by cap . If u don’t get it treated infection will may reach jaw bone and can cause swelling also in gums so better grit it done ASAP