Yeah! I would say you got yourself some double vertices.
The 3 main things that cause beginners problems are Double vertices, Flipped Normals, and Non Uniform Scale.
Martin has given the fix for Double Vertices…
For Flipped Normals – That is when the inside face is on the outside – Select All (with A) in Edit Mode and SHIFT+N to recalculate Normals.
Omar mentioned scale…
For Non Uniform Scale – When the scale in the Sidebar Panel (N) is anything other than 1.000 for X, Y and Z – With the Object selected in Object Mode press CTRL+A and select Scale.
These are the go to fixes if you notice strange behaviours.
