Might be teeth, might be a developmental leap, might be a latent ear infection, might be getting close to being 2 years old.
You may want to try giving some pain meds about 20 minutes before naptime and bedtime and see if it makes a difference. If so, you have a hint. It might be teeth, but might also be ear pain, so good idea to get checked out if this does make a difference.
I found and still find that my girls go thru sleep heck (and so do we! LOL) when they go through developmental leaps. Sleep goes to bits for maybe 4-7 days and then BOOM, a new skill emerges or a new bit of personality peeks through and things work back to normal.
One other thing it might possibly be: Josie gave up naps just past her 2nd birthday, but Natalie didn't wait that long; she was mostly done with them, barring an early wakeup in the morning or illness, around 18 months. You may be approaching that point as well. No guarantees, and one they get there they sometimes go back to napping again for a while, but it IS a possibility.
Keep us posted.