Its neither cold sore nor an insect bite.It is most likely a 'mucocele' or mucus cyst.
There is no specific reason for their sudden appearance.However,they go away at their own after a few days.
It may rupture after a few days.
Lip trauma can be one possibility like in kissing etc.
Which can result in their formation.
You should wait for 3 days .If it stays same then must see another doctor.
No need to apply anything on it.
You can apply some ice on it like for 2 minutes only.
Hmmmm...If you are worried you can see your doctor now.However,I think there is no harm in waiting 2 more days .
Try applying ice three times a day.
No they are not dangerous nor communicable.