You'll be absolutely fine, have no fear. It'll be cooler than high season (25 or so, today is 36 and too sticky to think), but still lots of sun. There's plenty to do and see on the coastline (Kotor, Porto Montenegro, Budva and Bar), with Podgorica and Kolasin easily rechable inland. There will still be a fair amount of tourists about then even though you'll be in the last few minutes of the traditional holiday season.