Might have gotten dry or might be burned due to chemicals in water or fertilizer. Hard to tell.

Try to keep the moisture as even as possible. Do you know the kind of clematis? Some tend to brown more at the bottom, but as BorS mentioned, it does look like damage after watering.

Thank you both. I haven't fertilized it, just watered with tap water. It's called Arabella. Could it be overwatering do you think?

You could be overwatering but that damage is not from overwatering. I could be from the nursery or store where you bought it. Damage to plants does not always show up right away. If you have fast draining potting soil and drain holes it is hard to overwater Clematis