Can a fever blister cause a swollen lymph note under chin?

Not cause, but: They could be related in some way. The virus that expresses itself as a fever blister can prompt your lymph node to work hard to attack the intrusion (usually an infection) and the effort can cause the node to harden and/or enlarge a bit. Also, it can be two separate problems. In any case, an exam may be wise to treat either or both. Feel better soon!

Fever blister: Yes definitely. Treat the fever blister and it will go away