Are you sure it was diarrhea or was it egg? Sometimes they start laying soft shelled or no shell eggs and they sometimes get it all over themselves. Often after that they stop laying altogether if they are going to have an internal laying problem. Has she been laying regularly? That will be the answer to that issue. If it is indeed diarrhea then I would start by deworming her. At her age I'd doubt coccidiosis unless she's new to your property or unless new birds have been brought in. It's not impossible just not what I'd suspect first.
I wouldn't give her any antibiotic's until you know what you are dealing with. One dose won't do her any good anyway. Corid is for treating coccidiosis. If you use the 20% powder the dose is 1/2 teaspoon per gallon of water for 5 to 7 days. You need to make it up fresh every day. But if this has been going on 3 to 4 weeks I highly, highly doubt coccidiosis. That moves pretty fast and kills quickly.
I would probably start with having a fecal test done on her and go from there. Especially if she has not been on a regular deworming program.