Mother tongue language lack of interest

The thing is that, from a year or so ago, I've realized that my interest for reading in my mother tongue language, that is, Spanish, has dramatically fallen. Instead, I prefer reading and studying in English. I don't know why, but I think that it has something to do with a change in my brain. I left Spain at the age of 4 and lived in Melbourne for around 7 years. So I came back at the age of eleven years old. Everybody knows that those are the years in which a child's brain is more open wide to learn new words and when it evolves according to the language the child speaks. That's the only reason I may find to explain why my school grades have been declining so dramatically since I was 17 years old (except for English subject, in which I always got excellent grades even without studying). I had excellent school grades, specially in physics and chemistry until the age of 17. But then I started to decline. I had real difficulties getting my bachelor in Business Administration and I feel as If I just can't read any book in spanish. If I could have any opinion from anybody, including any psycologist or medical professional it would be great. Right now I'm willing to move to UK to work and continue studying permanently over there. Thank you.