It could be a hematoma or bruise resulting from a traumatic injury, or insect bite.
The fluid acts to protect the injury, by cushioning the wound. It can be painful due to inflammation. But, just as you should not pop a blister please don't stick a pin in your dog's injury. Please take your puppy to a veterinarian and have it evaluated, especially since the injury has occurred on the head in such a young pet. Since bones are softer in the very young there is increased chance of damage to the brain and other soft tissues.
No - do not stick anything into the lump. You don't know what's going on in there and you could introduce bacteria through a puncture like that.
Think back - did he get injured yesterday, a hard thump on the back or a fall. I've seen hematomas (a blood filled blister) come up quickly from an injury.
A trip to the vet would be best especially since it's causing pain and growing so quickly.