If they feel around 1-2, then ask your vet to run a fecal float.Thank you so much, that's really helpful! They've usually had a nice amount of muscle (around a 3), but all of a sudden they're much thinner... I'd say somewhere between a 1 and 2.
You switched feeds, are they eating the new feed o.k.? Have you noticed if they are picking/choosing different pieces out and leaving the rest? Are you giving treats too?
With loose feed like that, let's say...they may pick out all the corn and wheat, but leave the peas or other things - if they are doing that, they are not getting a complete balanced "meal". At 16% protein if you are giving them a lot of other treats too, then that would dilute the protein and/or nutritional daily content they are getting. While this may not be the cause of weight loss, it's something to look at.