When calculating cash flow it's a lot more than just rental income minus your mortgage payment. You'll need to account for capital expenditures (roof, HVAC, water heater, furnace etc), repairs (little stuff that breaks like a sink, toilet, light fixtures etc), turnover expenses (when your tenants move out you'll probably have to paint or make other small repairs to get it rent ready), vacancy (if it takes a month or two to find a new tenant) and possibly utilities.
You might make $500/month for a year or so but when a renter moves out and you need to paint and replace carpet before you can rent again, that "cash flow" dwindles. It can evaporate completely when you factor in the cost of a new roof or other major CapEx's that will eventually come up.
BP has a really helpful calculator to help you run all the numbers. https://www.biggerpockets.com/buy-and-hold-calculator/new
Check it out and see what you come up with. Make sure you have property taxes and insurance accounted for if you haven't already. It may make you money in the short term but if the rents aren't enough to cover the true expenses of the property, it could have a negative cash flow.