Well, just about anyone can sue anyone else, including a former attorney, for just about any reason, including legal malpractice.
However, the fact that you accepted the settlement likely will work against you in any such case. Nevertheless, the settlement likely was between you and your former employer and/or its WC carrier. It likely was not between you and your former attorney. In other words, the settlement terms are unlikely to prohibit you from doing this. Of course, without being able to read the settlement documents themselves, we can't know that conclusively.
Obviously, we do not know how strong of a case you may have. I would suggest that you review your options with another attorney.