Heroin Very deep vein, crook of elbow?

Hi everyone. I hope you are OK.

I recently have had a slip up, and am again using heroin IV. I was smoking, until around a week or so ago, as I thought the veins in my arms had gone for hitting up.

I had a blood test though, and I went through the usual 'Sorry, it might take you a while to find somewhere', as usual, but to my surprise, instead of lining the needle up at a very shallow angle, after feeling around the nurse put the needle in at a near 90 degree angle!
To my surprise though, she managed to draw blood from me, and afterwards I had that terrible but great feeling people who inject know- I have a good vein!

I didn't have to mark the place, as I could see the dot where she went, but I have now used that deep spot a fair few times. I use the UK one hit kits from the exchange, which are 29G, 12.7MM 1ML needles with a filter.

I am just a bit worried how deep I am going in the crook of my elbow, but I have not experienced any pain and each time has worked well.

I just am unsure why exactly that vein is so deep? Is it from past abuse, and I thought it had gone, but it had simply gone deeper? Or was the vein always that big, and I can only now access the deepest part?

I have looked at vein maps and it seems I am OK and hitting the vein most people do, but having to go in at nearly 90 degrees makes me wonder if it IS that vein? Is there any more risk going deep than there is with standard IV injecting?

Thank you all for your help.

I know it's an old thread but others might Google this and find it so I'll go with my personal experience... I shoot in the crooks of my elbows and with both veins I have to go at a very steep angle with the whole half inch needle to get in the vein there.
My veins are deep it seems there. I can't see those veins in the crooks but I found them by feeling for them exclusively. Once you know where they are they can be hit reliably all the time.
So, no, it's not that strange that your nurse hit the vein at an almost 90 degrees angle.
It's not a deep vein because you aren't hitting it with a long needle. It's still half an inch max below the surface of your skin. Deep veins are deeper than that.

If I use my thighs I have to go at a very shallow angle instead as veins there are very close to the skin.

I find the superficial veins on the thighs easier to hit than anywhere else on my arms as my veins seem to roll so much even with a tourniquet.
I can see many veins as I've got visible veins on my arms and hands but cannot hit them very easily. Also, using the thighs avoids the issue of visible bruises and so on.
They work fine for me when I wanna give my crooks of the elbows a rest.
I still have all my veins as I've only been shooting up for a couple of months or so now.