"I wasn't playing football" may indicate you were doing something else "I did not play football" may indicate the same or it may also indicate that you played another sport. Normally in daily conversations a native speaker would supply the extra vague information, if required. Much would depend on how the question was asked, if it is a question that is being asked. "I wasn't playing football, I was at my mothers house" "I wasn't playing football, I went to the seaside" "I did not play football, I played cricket instead" "I did not play football, I could not make it" = you could not get to the playing field or get there on time. I wasn't playing football may be used more often when you were somewhere else or doing something else. I did not play football may be used more often if you could not play football. But there are difference in use by different speakers so it is usually not worth worrying about. You can use any version and explain or explain if asked to explain.

No there is not a huge difference. I wasn't playing football would perhaps indicate that I was playing something else. E.g. I wasn't playing football, I was playing cricket. Equally I didn't play football may indicate that it was expected of me, but I didn't do it ... so, I went to the park, but I didn't play football. But in everyday language they are interchangeable.