Also watch out for general ill/under the weather feeling in urself, that’s another sign of infection.
It sounds like a blocked duct that can lead to mastitis if not sorted asap. I've found the only thing to clear mine in the past is a combination of warm compress, gentle massage (especially when baby is feeding), ibuprofen and epsom salt soak where you literally mix some epsom salts in a bowl of warm water and dangle your boob in for 10 mins 😂 sounds ridiculous I know but I've had my fair share of blocked ducts and doing those on repeat has always cleared it within a day. Good luck!
Have u tried massaging the mass while the baby is drinking? The lump sounds like a blocked duct…Having experienced mastitis I would say the minute it starts to be painful to touch/breastfeed is when u will want to speak to the doctor to get some antibiotics as it could be infected. Otherwise for now they’d probably recommend doing what ur already doing 👍🏼