Chain dropping?

[neilbardsley]

I'm having an issue on my CX bike. Which is shimano with 38/48. On the last 2 rides when I'm in the lowest gear on the cassette my chain drops off the small chain ring. It doesn't happen on the 2nd from lowest gear on the cassette.

I decided that maybe my rear mech was limited enough so perhaps was pulling the chain off at an angle. Therefore I played with the Lo limiter screw and indexing adjustment to ensure that wasn't the case. On on my bike stand it was fine but once riding the bike again it dropped? Given that it has only just started happening could be be wear related? My chain is becoming loose? If that is the case then why doesn't it drop on on the 2nd lowest gear where the chain has more slake? Perhaps my chain ring has worn so it is easier for the chain to drop off? Maybe i need to adjust the angle of the rear derailleur to the cassette? Any help is appreciated.