Random screaming during breastfeeding?

Sometimes when I'm breastfeeding, my baby (4.5 weeks old) will be eating fine, and then she'll suddenly scrunch up her face, get really red in the face and start screaming. My first thought was that it was gas, so I always try burping her, but it doesn't seem to help. She will usually latch on again for a few seconds to a minute, but then the same thing will happen again. It tends to happen more on one side than the other, but it has happened on both. Has anyone else experienced this? I don't know if it could be acid reflux, gas, or something having to do with my milk...I've also tried different positions to see if it is the latch, but it still keeps happening.