My baby girl (now almost 6-months-old) did the same thing. I want to say it started somewhere around 4 weeks, too. She would only eat for about 5 minutes and she also had a problem with one of my breasts. She would often cry at this breast or refuse to nurse from it. She would also have days where she just didn't seem to be eating much. It really worried me (I called her ped a few times about it and took her in to see him twice). Thankfully, she has been gaining weight just fine all along so I made my peace with her habit of snacking. Does your babe eat often? Because, my LO had very short feedings, but (on the days there were no hunger strikes) she ate very often. She continued to nurse this way (short feedings, very often) until she was about 5 months. She now eats every 3 hours pretty consistently and for closer to 10-15 minutes. I think it was just her nursing style to be a snacker for so long. I also notice that she is capable of taking the milk from my breast very effectively and quickly now so she doesn't need to feed for very long (she gulps it down like a mad woman:).
As for the breast she often refused in those earlier days, my midwife told me that it was likely one of two things—either the milk was coming out too fast or not fast enough (both situations can easily frustrate a baby). I used a breast pump to figure it out and sure enough the milk was coming out like crazy from the breast my LO was refusing. It became very obvious because she would suck only until the milk let down then she would pull away and milk would be gushing and spraying out of me. She had trouble handling it for the first few months of her life. The older she got, the more she preferred this breast, though. Also, my milk supply leveled out as my body adjusted to her needs so that one breast wasn't as overwhelming as before.
So, in our case, time was all we needed to get used to eating in a more "normal" way. But, I will say that it's still important to make sure there is no medical issue that is causing the short feedings/refusing one breast. Listen to your trusty mother instincts and if you feel like something could be causing this, take her into the doctor. I went through all the possibilities—gerd, acid reflux, allergies/intolerances to things in my diet, etc. I did an elimination diet when my baby was 4-weeks and did discover that she couldn't tolerate cow's milk in my diet. Although, removing that from my diet did not change the short nursings (it was the cause of colicky behavior, though).
The fact that your LO is gaining weight, has plenty of wet diapers and a few poopy ones is a good sign, though. I know that there is no "normal" when it comes to how many BMs per day they should be having. Mine was having about two and also the small ones that don't really count and her ped said that was totally fine. Anyway, long post. I don't know if any of this helps, but it's always nice to know someone else had the same issues, right?