So my friend dragged me to a bras shop and insisted on buying a new bra! (she also insisted on a push up)

She wanted to try fit me with a certain underband, the woman at the shop said it's too loose and I need a smaller one.

After a while of adjusting the cups, they stood and said "woooooow" deciding it's a good fit!

Problem!
Even though I don't feel discomfort, at the end of the day (and I checked, it doesn't happen after 30 minutes or an hour I think), I have marks in the shape of the bra wearing it.

I was told to put my fingers inside the underband and pull it, see how much I can pull it away, if I can't pull it much I might need a bigger size.
Apparently, I can pull it quite a lot away from the body suggesting I need a smaller band size, I am confused !
At the shop, they were convinced one size larger was way too loose, she didn't even need to pull it to test it.
Now I am having red marks and can still pull it quite a long distance away from the body.

Can anyone make sense of this?

I am assuming this is an under-wire bra. They will do that if the under-wire is the wrong shape. In the older style with metal under-wire you can bend the wire into the right shape for your breasts, but the newer bras have plastic under-wire, and you can't do that.

Bra's (or as I like to call them, torture devices) can leave some lines -- same thing if you lay on a sting while sleeping, lines show up. If it's comfortable, that's good. You don't want too loose of a band, or your bra will move around on you. Too tight and it will hurt (cup or band size). Either way, I find by the end of the day bra's get uncomfortable. o_0

Bras only hurt if they are the wrong size. However, it is not unusual to have some marks from a new bra. They need to break-in just like shoes. They should stop leaving marks after 2-3 wearings and/or a wash or two. Note: you should not wear the same bra 2 days in a row. They'll stretch out too much that way. Alternate between two bras to give them time to regain their proper shape on the off day. They'll last much longer this way. Hand washing is also recommended.

Just my experience and the advice of the place I buy mine

WR

There are also differences from one person to another, some get marks easier than others. For me it has been an issue since about forever. I get a rather nasty red band around my forehead every time I wear a hat for more than about a quarter of an hour, and usually visible marks on my wrist from my watch strap within a couple of hours unless it's loose enough that the watch can rotate completely freely around my wrist.

People see me as male, and for various reasons I am mostly going along with it. I do, however, have breasts somewhere in the 80A range (no inch sizes here  ) by now, and they are sensitive enough that I feel more comfortable with some sort of a bra, normally a simple cotton top (as I've found some that actually remain comfortable all day). Even these leave marks, so that I take my bra off a couple of hours before going to a swimming hall -- perhaps these new body parts of mine will eventually grow big enough that I'll have to consider a swim suit that covers them, but at the moment that'd look sillier than going topless.

Sounds like Myself just has a skin that gets marked easily, like myself.

  Nfr

80A is 36A
I am not sure, but I think it's the underwires, when I wear one without underwires I have no marking.

Alright. So, I'm a week late on this, but at least I have experience wearing bras. 

Underwire bras; in my experience, old or new, leave marks at the end of the day. So long as you're not getting discomfort from them, this isn't abnormal at all. If you can toss your arms up over your head, carry stuff around, and do your daily activities without being constantly reminded of the fact you're wearing a bra, then it fits. If it's too big, it'll shift around, and you might pop out the bottom. if it's too small, you'll have discomfort and possibly chafe-marks or blisters (bad) around the ribcage where the band fits.