My dog has bumps underneath her skin... what are they? and are they serious?

[puppyfaces234]

So for the past couple months I've noticed that my dog has these big bumps underneath her skin... they're like 3" in diameter (it varies between each bump... but that's about the average size), and they're spread like all over her back/sides. They're kinda squishy... and she doesn't really like it when I'm checking them out (like when I'm touching them to see what they are) so idk if that means it hurts or she just doesn't like it. She's about 8 years old, and she gets fed some people food every now and then ---but mainly like meat and stuff... I'm just worried 'cause I'm not sure if it's something serious like a tumor or w/e... please help! Thank you!!