Is it okay to have another lip filler session a week after the first one or is it too soon?

I recommend that you go for a follow up with your injector or if traveling or away visit another board certified physician and expert that can give you his professional opinion. Follow up or new evaluation is necessary beforehand for proper recommendations to make sure you are not doing too much too soon before properly healing.  
Best and thank you for your question,  

Thank you for your excellent question.  Swelling is normal after treatment with fillers, especially when the lips are injected.  Please keep in mind that it will take 2 weeks to allow for normal postprocedure swelling to fully resolve and to see your final result.  For this reason, two weeks is a good time to reassess and add more product, if necessary, to achieve your desired result.  Hope this helps and best wishes!