Hi this is called a mucocoele.
Usually occurs if you have a lip biting habit or because of a sucking action of the mucus membranes.
It is usually painless as you described.
Its nothing to worry about, does not require any treatment and will resolve on its own.
Meanwhile you can follow the instructions given by your dentist.
Next Steps
Since you mentioned this is second time you are getting it, if it keeps recurring then the treatment options would be to surgical intervention, laser or cryosurgery.
Health Tips
Please refrain from biting your lips if you do that.