I am sorry to hear that your dog is limping. There can be many different causes of why a dog might limp.
Even though he may not be showing any obvious signs of injury he could have damaged soft tissue such as muscle, tendon or ligament which may not have been dramatic enough to notice when it first happened.
I would advise seeing your vet who will probably ask a series of questions as well as perform a full examination to try and determine the root of the cause.
Your vet may ask things about when you first noticed the limp, which leg/s he is limping on and how long has he been limping for. By carefully watching him move and examining him they will hopefully be able to pinpoint where the pain is coming from.
Certainly if his nails haven’t been trimmed in a while- they can sometimes grow around and start to embed in the paw pads which can cause discomfort and sometimes progress to infection. Your vet will be able to do a thorough check of his feet and paw pads to rule this in or out.
I hope he is back to his normal self very soon!