Muscle building

Hey, I got to the gym around 3 times a week mainly doing weight training and use my dumbells at home, with some added sit ups etc. I used to play football for a team for 9 years but quit 2 years ago. I want to get into some more stamina training and football training on top of what i do now. Will that cause me to lose any muscle?
Thanks

Pete 17 years old
 
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