So my little man hasn't been feeling that great the last week or so, it started out as a cold but…

So my little man hasn't been feeling that great the last week or so, it started out as a cold but then for a couple days he had a fever but it went away. Today though he's just miserable, he's crying and whining out of pain in waves and I don't know why. He's been teething but it's never affected him this bad and he doesn't have a rash... Any idea what might be wrong?