Traditionally, the kriat shema was said once in the morning and once at night (BT, Ber. 10b). For night time, i.e. going to bed, a distinct blessing was formulated - ha'mapil (ibid. 60b). Naturally, as the two pieces were unique for the evening they eventually took form as a unit, recited prior to bedtime. After the Halacha was codified, two orders were popularized (cf. order of Rambam, Tefillah 7:2, Shulhan; Aruch OC 239), one was to say the KS first and then ha'mapil, the other was to first say ha'mapil and then KS.
As it appears from your question, you follow the former. Therefore, to recite the KS (characteristically known as Kriat Shema shel arvit) before bedtime is fine since it is still a proper fulfillment of its evening recitation (cf. Aruch Ha'shulhan §239:5) and ha'mapil -in your case- is likewise still in fulfillment of the original enactment since you are reciting it at its proper time too, i.e. before bed.
In my opinion, there is no issue -in this scenario- of "hefsek" (interruption) because KS shel arvit is not unique for bed since it was prescribed for evening, not bed. (Although the Talmud (BT, Ber. 4b) cites a prescript that it be recited when in bed, I do not believe it was meant literally, as a requirement for exactly that time, because the KS isn't unique for sleep but rather for the phenomenon as opposed to ha'mapil which is unique for sleep See Meiri Ber. 60b.) The only debatable issue of interruption would be if between a blessing proscribed on sleep and the sleep itself (or preparation of it) one were to interrupt with unrelated musings. In any case, consult YLOR for actual practice.