You might want to check with your doctor to be safe, especially if the pain is coming in regular increments. When I was at 32 weeks with DD, I had pain that shot horizontally across my lower stomach. At first I didn't think anything of it, but then it kept happening. Turns out I was having pre-term labor. I got a shot in the butt, went home, and DD ended up 2 weeks late.
With this preg, I too have had vertical lower stomach pains which my doc says is just round ligament pain.
I had shotting pains that went in the verticle direction down low on my belly off to one side or the other. Dr said they were round ligament paint. It usually happened when I was getting up or laying down. It only happened occasionally.