I just mention this as my horse Jet does this every Spring. There is nothing wrong with him except weight fluctuation. He is fatter than normal over the winter, take the blankets off and he drops a bit and a heave line shows for a couple of weeks until the weight goes back on. Maybe it could work the other way around?? I don't know, I'm just speculating.
My horse has the same thing, heave lines but is also not eating his hard feed, just grass, and is laying down a lot. Had vet out twice, given bute etc but no vast improvement at the moment, they thought it might be viral. Interestingly though, I think we have rape seed next door to us.