Ants on a potted lemon tree

I have recently noticed a few curled up leaves on my lemon tree and on more closer inspection have found ants at the base of the tree this morning! This tree was repotted into this larger container 3 days ago when the ants were not present. Doing a bit of research I am wondering if what I am occasionally finding on the leaves are aphids and could they be the cause of the ants being present now?

I had bought a young potted kumquat tree that is next to the lemon tree also which I am thinking may be what introduced the pest? Thank you for your time reading this post.