It is called a pregnancy epulis. Basically in pregnancy the increaed levels of hormones in the blood stream make the gums more susceptible to inflammatory change. This characteristically causes the gums to be red and swollen and prone to bleeding. In severe circumstances it can cause these lumps known as epuli which can increase in size at quite an alarming rate. Genenral once the hormone levels have settled down after the birth they resolve by themselves. Occasionally (as in your case) they don't so need to be treated surgically (ie excised - meaning local anaesthetic injected into the area to numb it, the offending piece of gum cut away and then a couple of stiches. The piece will probably be sent for pathology/histology to check that it was a pregnancy epulis, and you'll be given post operativeinstructions on what to do/not to do to make sure it heals up without probs, including what pain relief to use, hot salt water rinsesto keep it clean etc. The stitches will probably be soluble so you won't need an appointment to have them removed. Plus I would recommend taking arnica 24hours before and for 3-4 days afterwards to help speed up the healing.
HTH