This doesn't happen to me often but I can think of two instances in the past. One was in hospital after DC was born and I put it down to BF hormones. Another was watching a film in bed.
I'm currently visiting with family and just sitting in the front room mostly reading Mumsnet while the tv is on and DC is playing. I have that sudden feeling of relaxation, which I get also if someone is cleaning around me (not sure why) but they are not.
Does anyone else have this and can you explain any of it?