Raising utility cost mid lease?

i have a question I’m having trouble finding the answer to. i rent in Denver and my lease says very specifically that I will pay X amount of dollars every month to cover the cost of utilities, however I received emails last month that they are no longer going to charge flat monthly fees and use a 3rd party billing provider to charge utilities. Now my utilities cost has gone up from a flat 73 a month to nearly 150. Is that allowed and is there a limit to how often they can change the cost?