6 week old isn't napping well?

I am feeding my 6 week old and she seems to always be alert and awake during the day. So much so, that by evening she is exhausted and it's very difficult to get her to sleep at all. I feed her when she wants during the day (which is usually every hour to hour and a half). Once we're done feeding and burping, I try to put her down to sleep and the second she loses contact with me, she wakes up. I've tried all the stuff that works at night (patting, rocking, swaying, etc) and she just gets mad. We get up every 3-4 hours to feed at night and get up for the day around 8am. She usually eats every hour until 11 or 12 and then will sleep for 2-3 hours. Then, she is up and I usually can't get her to sleep longer than 20 minutes the rest of the day. Any ideas? We've been to the doctor and nothing is physically wrong. Recently, she has decided that she hates being swaddled--she has become very active the last week. She can support herself on her legs, hold her head steadily when she is upright, and can lift her head 45 degrees when she is on her tummy.