Nap time... or is it just scream time?

My little girl is growing up so fast! She is 5 months old already. The battle of the moment is nap time. She doesn't really have a nap schedule. She likes to take a nap in the morning, and one in the afternoon. As for specific times, what works one day will not work the next. That's partially my fault, as I take her out a lot. But these days, she will act like she's tired (yawn, rub eyes, etc) so I put her down for her nap, and she SCREAMS! This is pretty much the only time she really screams like that, except for when we are at walmart and she decides she's tired.... haha. So there are some days that I have to pick her up and try the nap time later. Lately, this is the case every day! I'm not a big fan of letting her cry it out, but sometimes I will let her cry for a few mins. She will even suck her thumb, close her eyes, and scream LOL Thank goodness this afternoon's nap went not too badly. But really, I have nothing to complain about. She eats every 4 hours from 8am to 8pm. She sleeps from 8pm to 7:30-8am, and has since she was about 3 and a half months old! She is the light of my life!