Confused about my flange size

I’ve been pumping 7-8x a day for 3.5 weeks. At the hospital they told me my flange size was 21 for the left and 18 for the right. I’ve been using a 21 insert for the left and 19 for the right as that’s what I have.

My left nipple is really sore and sensitive. Like when fabric brushes against it it feels like pin pricks.

I’m starting to experiment with different sizes to see if that’s the problems but I feel like it’s a lingering problem so might take time to figure out what is wrong. I’m confused if it’s too big or too small. It’s hard to tell if the areola is being pulled in bc my nipple kind of has a weird transition into the areola. When I start the pump the nipple doesn’t rub against the sides, but about 5-10 minutes in, it swells up and is hitting the sides of the tunnel. I use nipple butter to help it glide, and also decreases the suction to where it’s comfortable, but it doesn’t really hurt during the pump, just throughout the rest of the day. Mostly it’s sensitive to touch.

Anyone experience this? I’m just confused if I’m going too big or small? I plan on talking to a LC as well, but I’m going by what the first one told me! Are nipples supposed to feel this uncomfortable?