I have been in dentistry for 15 years now and we see many patients with these little hard lumps, which are blocked salivary glands. They are no problem as long as they clear themselves, although sometimes you can get little stones forming in them, but this would be very painful and the "bump" just wouldnt disapear.
I have one that appears on the roof of my mouth from time to time they can feel quite sore and very annoying as your tongue tends to be drawn to any thing like that in the mouth, but it really is no problem.
As long as they clear up things are fine but if you have one that lasts for over a month i would go to see your dentist.
hope this helps