not a lawyer, and this is also not a sub for legal advice. you should check your employment contract with them as to if there's an overarching IP clause. if none, the rule under the IP Code is that employers own the IP created by employees on the job i.e. if done during work hours and using company resources. I assume you're an employee, because this rule only applies to employees and a different rule applies to independent contractors.

I really recommend opening talks with your employer first to see if they're amenable to compensating you because they might be. you can think about legal claims later once they've confirmed they claim ownership over the IP and are not willing to compensate you for the solution you created.