Help me understand sleeping patterns

Stacy

My baby is 15 weeks old. She takes 45 minute naps, although sometimes a 2 hour nap will sneak in there. But generally our day starts around 7am. She will play till 9 - this is her longest awake time. I'm usually finishing rocking her and putting her down drowsy but awake by the time the sitter arrives at 9:30. The entire day after this is 45 min nap, 1hour play, 45 minute nap, 1hr play... when she wakes up from her 45 min naps she's all smiles. Then after being awake for 45 minutes you can tell she's tired and hitting a wall. As you can imagine my baby takes a lot of naps between waking at 7 and bedtime at 7:30. Just recently she's started to REALLY fight sleep. She cries and cries in our arms like she's so tired she cant handle it. Where am I going wrong??? Night time feedings usually happen at midnight and 3:30 am. She doesn't cry herself to sleep for those, she just eats and goes to sleep wonderfully.