Needing to help stool out with a finger after a sudden change from your usual pattern, especially with your history of IBS and GI ulcers, suggests the rectum or anal canal isn't emptying fully on its own right now. This can happen with a stricture from old ulcers, weak coordination between the rectal and anal muscles, or a partial blockage, rather than plain constipation.
Next Steps
Please see a gastroenterologist in person soon, who can do a rectal exam and possibly anorectal function testing, rather than trying more fibre or over-the-counter options on your own given your ulcer history.
Health Tips
Seek care sooner if you develop bleeding, fever, inability to pass gas, or worsening abdominal swelling.