Pulsating/Throbbing back pain!

Sarah
Went for a growth scan yesterday, baby is approx 5lb 7oz at 35 weeks. Sonographer couldn't believe how low the babies head was in the birth canal. Said she's surprised I'm not walking around like the baby is about to fall out.
Now I didn't ask her as didn't think at the time...I've been getting a throbbing, pulsating pain in my Lower back on and off. Probably happens once or twice a day and happens in almost waves of nothing, throbbing on and off for a few seconds, back to nothing, then repeats, only repeating twice pretty much immediately one after the other.
Has anyone experienced this? Could it be slow labour? 
I'm back at my midwife Wednesday. I did mention this to her last time but she said it sounds like back ache due to babies position. 
The more I think about it the more I'm thinking it may be something more. 
Any advice would be appreciated! 
TIA

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