Just buying a new bag isn't the worst thing that could have happened since you will have to taper your cat off the old food and onto the new food slowly to avoid GI problems. I also just recently switched my food from blue after learning about it in my vet tech classes. I switched to Hills science diet indoor formula for ages 1- 6(not the one required with a prescription) there is a lot of BS floating on the internet about pet foods and its easy to get confused on which ones are actually good for your pet. If you decide to go this route amazon is the cheapest seller by far.
The lump could just be scar tissue. Humans get that too. It is only a problem when it gets tied around organs or gets in the way of mobility. Still have it checked but it's probably not anything to worry about.
As for the food. I would suggest looking for something else. The key you should look for is when you read the ingredients list, protein should be first on the list. Either chicken, turkey, beef, etc. 9 Lives starts with corn.. that is a bad sign. Every owner really has their own preference for food but that is your first indicator of what food is good for your cat. My vet suggest https://www.amazon.com/Hills-Science-Diet-Indoor-Chicken/dp/B003MA8P02 but my cat just doesn't seem to like it. Probably because she doesn't seem to like chicken. Go to Walmart or wherever you shop for food and just spend a good 30 min looking at all the ingredients lists for the indoor formulas. Look for ones where the protein source is listed first.