Sounds very much like these "lumps" are in fact haemorrhoids. Sometimes they are referred to as piles.
The doctor will probably examine them and it is likely that he will prescribe Proctosedyl suppositories and a cream to apply to shrink them.
Sitting at a computer desk for a long time can cause all sorts of muscular skeletal problems, so do get up and have a walk about or have a stretch at least once every hour. If the seat is a problem, you should inform your employers to provide suitable seating for the job that you are doing. This will come under Health and Safety Legislation.
This sounds like an internal hemorrhoid. It can pop out & make an appearance after having a bowel movement, it can feel like a hollow soft round bump, I guess yes you can say it almost feels like it is filled with air. Usually it goes back inside the anus after going to the bathroom, however sometimes after a while it may stop doing this on its own & you may have to manually push it back up inside, sounds gross but it really isn't. I have had one of these for many years, it got worse when I was pregnant with my son & I've just gotten used to it. It can be very frightening when you find it for the first time but it isn't commonly life threatening and most of the time not painful, internal rhoids tend to not hurt as much as external rhoids. It usually doesn't bleed unless irritated-i.e; if you have diarrhea or constipation & straining. Just make sure if it stays sticking out that you push it back up inside. Rarely this can be some kind of prolapse but most of the time it's an internal rhoid. They can be flesh colored and not always the stereotypical red or purple looking rhoids you always hear about, mine is the color of my skin. I'd say it's been there for at least 20 years. If it still worries you you can have your doctor take a look at it but I wouldn't stress too much over it, doesn't sound like you have anything bad or life threatening. A lot of people have this.