Agree with snailplane.

Making it more clarified, as I read on a few boards, since preferably takes 'has/have' to state that the situation has remained from that point of time. 'After' dose not.

So,

The situation has become worse since ...
and
The situation became worse after....

Agree with Frederik on 'gotten' as I see in Oxford.

This question's answer is close and may be useful.

Agree with snailplane.

Making it more clarified, as I read on a few boards, since preferably takes 'has/have' to state that the situation has remained from that point of time. 'After' dose not.

So,

The situation has become worse since ...
and
The situation became worse after....

Agree with Frederik on 'gotten' as I see in Oxford.

This question's answer is close and may be useful.