First, the tire will flat bottom when left unmoved for several months. This normally corrects itself after 10 to 30 miles on the road, but the road will be bouncy until then. A tire should be rotated every few months to reduce the flat bottom effect.
Now to the answer to your main question, more likely loose bearings. With the tire mounted and off the ground, the wheel should spin freely with very little to no inward/outward movement. Given someone self did the did the previous work, you should redo the work yourself.
In my case, I found the previous owner was cheap with the grease. You should do a full brake/axle inspection and replace the bearings. Remember to replace the cotter pin as they break after one or a couple uses.