Please show the full Row Source statement. Use Zoom if needed.

This is normal behaviour if the Combo Box is bound to an ID.

To be able to Add/Edit your Lead Sources set up a List Items Edit Form

What baffles me is that it worked perfectly and then all of a sudden it gave me the message

As Kevin R. said, this is by design: if the bound column is anything other than the *first visible column*, Limit to List is automatically set to Yes and can't be set to No.  It worked before probably because you were under a different circumstance.  Your screenshots show that your bound column is hidden, and that's why Limit To List must be Yes.

Examples of how Limit to List works:

1st column: hidden
2nd column: visible
3rd column: hidden
Bound column: 2
Limit to List can be NO, because the first visible column (the 2nd column) is bound.

1st column: hidden
2nd column: visible
3rd column: hidden
Bound column: 1
Limit to List must be YES, because bound column is hidden.

1st to 19th columns: hidden
20th column: visible
Bound column: 20
Limit to List can be NO, because the first visible column (the 20th) is bound.

1st to 20th column: visible
Bound column: 10
Limit to List must be YES, because the first visible column (the 1st), isn't bound.  Only if Bound Column is 1 can you set Limit to Yes to No.