If the injury which force you to take a month off is true, it could be serious. You should have seen a doctor.
However a month off isn't so bad as you think, muscle-atrophy-wise.
As for strength, once your CNS and PNS are online again your strength will soon come back. So there's no reason to rush.
Now cautious part has been said, I'll give you a real advice.
*Just stick to "not to failure" for at least a month.
When you do a exercise at its full ROM, you watch (I mean feel) your shoulder very closely.
Remember there's no need to rush. Once your injury is fully healed, you will regain mass and strength in no time.
Don't make the same mistake as we older ones did. *wink*
Good luck!!