F,
yes you should be concerned anytime your pig does not exhibit its normal behaviour, but don't panic
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Presuming that 1 pig of 2 is normal, then it is unlikely to be environment (eg something harmful growing in their pen).
I presume it is not physical - eg leg strain.
Pigs do have off days, so as he is drinking I would monitor for the next day or so, he won't die through lack of food as long as he is drinking ok.
If he does not recover to normal self for 48 hours, then take his temperature. This is quite easy - have a digital thermometer and as he is lying insert into his anus. Pigs are generally very tolerant of this, and with a digital you simply wait until it beeps and then withdraw to read it at your leisure.
Do this a couple of hours apart. If you do phone your vet, this is key information he will need - ie is temp normal, and is it going up or down or stable.
If he does have a temperature (Normal temperature 38.6ºC to 39.5ºC ), then vet is likley to propose antibiotics. If temperature is normal then antibiotics will do not good, and it is more likely that he has had some trauma or eaten something that has upset his stomach - time will normally heal.