If you're only having pain in the beginning that goes away (and doesn't last for the whole feeding), his latch might be ok.
I wouldn't worry about doing both breasts at each feeding unless you need to for your own comfort. Typically, people start the feeding with the breast they ended the last feeding with (if they can remember).
Can you post her weights at different ages like this:
4 wks: ____
6 wks: ____
2 mths: ____
That would help us picture it better.
How many times a day is she nursing? For how long?
How many wet and dirty diapers is she putting out per day?
I recently put a password on our kids' laptop so that they can't get onto the computer without me logging them on. 12yo ds was spending lots of time on the computer and he started getting up before us in the morning so that he could get in more (unsupervised) computer time, so I felt that it was time for more limits.
If it has been a while since the last time she nursed, I wouldn't wait to pick her up and feed her, since crying is a late sign of hunger.
If she has just nursed, it's nighttime and she's fussing a little bit, I might wait a minute to see if she goes back to sleep.