Feel inside her vent about 1-2" for an egg.
If she's able to poop, then there's not an egg low enough to be "binding".
When is the last time she laid an egg? Feel her abdomen below the vent between the legs - any bloat or feeling of fluid?
Well if she's vaccinated then it should not be Mareks I would treat for egg binding, if you can afford, take her to the vet, if she isn't egg bound, try googling her symptoms.
I'm sorry, but that is not how the Marek's vaccination works. The Marek's vaccine can
help prevent the formation of tumors but does not prevent infection from the virus. Plenty, plenty of folks have birds that exhibit symptoms due to Marek's disease (yes, they have sent birds for testing/diagnostics and had confirmation of disease). It's not uncommon to see birds exhibit symptoms of paralysis, wry neck, etc. even if they have been vaccinated.
To complicate matters - symptoms can come and go - the disease is extremely heartbreaking and frustrating for backyard keepers. There are 4 forms of Marek's disease - Cutaneous (which affects the skin), Neural (which involves the nerves), Ocular (which involves the eyes) and Visceral (which involves the organs). Symptoms from each "form" could also be seen. The only way to know for sure if a bird has Marek's is to send for testing.
Here's an interesting read for everyone to read about Marek's. It can shed some light on how complicated it can be and also highlights conditions that can look like Marek's as well.
http://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq