Aspirate. I didn't think anything was back there either till i withdrew and blood flew 4ft and hit my bathroom wall. But that 10 years ago and i was dumb. Now i won't think about injecting without aspirating.
Oh, keep hitting your ass only and not rotating and you'll eventually build up enough scar tissue that if you hit it, you won't be able to inject. You'll have to withdraw and find another spot. Yea, i have that problem sometimes.