Baby won't sleep!

My LO is almost 6 weeks old and most of the time she takes cat naps during the day (30-45 minute stretches) and at night sleeps only about 1.5-2 hour stretches. Sometimes she feeds for like 5 minutes and goes back to sleep on my breast but I think she just wants me to hold her. She sleeps well when she's held but as soon as we put her down she's up and crying. For a few days there she was sleeping 4.5-5 hour stretches at night and 3 hour stretches during the day so I'm not sure what happened because I haven't changed the bedtime routine. I need suggestions!