Mom has a lump. It does hurt her. I'm concerned it could be a bed sore. Any suggestions?

It may be a bedsore and it could be a cyst or infection developing. Neither will be fun or easy to heal but based on the area and the fact she is sitting in that position close to 24/7 by the sounds of it infection is a very real possibility and could become a real problem so I would encourage you to get it looked at asap.

Make sure her doctor really understands that she is spending both day and night without much movement in that chair too, hopefully he or she will be able to impress upon her the pitfalls that may create.

According to webmd website:
https://www.webmd.com/skin-problems-and-treatments/pressure-sores-4-stages#1

What you are describing sounds like a Stage 1 bedsore or pressure sore.  "Stage 1: This is the mildest stage. These pressure sores only affect the upper layer of your skin.   Symptoms: Pain, burning, or itching are common symptoms. The spot may also feel different from the surrounding skin: firmer or softer, warmer or cooler.   The spot doesn’t get lighter when you press on it, or even 10 to 30 minutes after you stop pressing. This means less blood is getting to the area."

Many times the underlying tissue is damaged first and then the skin "breaks open" to reveal the bedsore.  I suspect that the lift chair is not fitting your Mom properly and that the lift chair cushion is putting pressure on her buttocks.  Or your Mom is rubbing her buttocks on the chair as she attempts to reposition to find a more comfortable position. 

Try sitting in the chair and see how comfortable you feel when you sit in it.  You may find that the cushion is uncomfortable in certain spots.  Gel or foam cushions with the buttocks area cut out are good suggestions as long as they don't cause pressure along the cut-out.

It may help for your Mom to eat a diet high in protein, vitamins A and C, and the minerals iron and zinc. These are all good for your skin. Also, drink plenty of water.

Since your Mom has a painful bump already, she needs to see her doctor ASAP before the bump becomes larger or becomes an open sore.

As well as getting your mom checked out pronto, you might consider getting her a memory foam or gel cushion that has a cut out area for the tailbone. My husband has very little rear end "padding" and gets uncomfortable sitting on hard surfaces r even on soft surfaces, such as his lift chair, for long periods of time. He uses one of those kinds of pillows and says it helps a lot. We have one for in the house and one for the car seat. He uses his car shhion when we go out to restaurants or to visit friends. It has the added benefit of elevating him slightly so it's easier for him to get up out of seats. This isn't the exact one, but they look like this: https://www.amazon.com/SOFTaCARE-Best-Seat-Cushion-Relieve/dp/B07CYNC65H/ref=sr_1_10?keywords=coccyx+cushion&qid=1556055396&s=home-garden&sr=1-10

You'll need to get her examined by a doctor or a very experienced nurse.

Her skin there probably gets very sweaty and this is probably some kind of cyst, but whatever it is it needs seeing to. It would also be a good opportunity to investigate whether there is some other problem with lying flat that she hasn't told you about, and to speak to her firmly about the importance of moving around and changing position properly. Because even if this isn't a pressure sore, this time, she's certainly going the right way about getting one.

Have her seen. A bed sore is worse under the skin. What is visible may not be the whole issue, if that is what it is.
Others can explain how to treat it until she can be examined.
Try adding an egg-crate matress or two, cut to the size of the chair.
Buy a piece at Wal-Mart.
The new lift chair is suspect....Have you sat in it?
Get her up and moving right away.

Recalling a caregiver who used chair pads in her Dad's bed to prevent bed sores.
He never had a bedside even though he was bedridden. Hey, Bookluvr!