You're not alone. I am having exactly the same problem and I'm stumped, I'm afraid. My little girl is just off 7 months and has slept like a dream until now. She is an angel with some touchy characteristics, so sleeps only in the dark in her cot and still swaddled (although she now has one arm out). She used to sleep from 9-11, then from 1-3 and then 5-5.30 and then right through the night. You could set your watch by her. Yet for the last three weeks, she's only been catnapping and I just can't get her back to sleep (At night she still sleeps very well - hope that continues, at least). Result? Grumpy baby who is not eating well. She's rolling around a lot now and that may be disturbing her, although I don't understand why that doesn't disturb her at night when she isn't swaddled. We've tried not swaddling her for naps (no hope), we've tried tighter swaddles (again, no hope).
She started resisting her 5.00 sleep, again about three weeks ago and so we've tried adjusting her routine to get her down to two naps and that hasn't worked, either. Will read any suggestions with interest.