Flashing lights

25+4 weeks. On two occasions over the past few  weeks I have seen brightly coloured flashing lights in front of my eyes. I'm not dizzy with it and It's not when I get up as both times it's happened when I've been standing up for quite a while. What could this be? Maybe it's normal pregnancy symptom? Never experienced this in first pregnancy. when I googled it Just said pre-eclampsia but I'm not swollen and I feel well so I doubt it's that. Especially this early. Not sure if I should phone midwife or just see what happens over next few days/weeks. Thanks for any advice