20 weeks pregnant and I experience this ??

You should get a check-up to find out if there's a problem. Get your blood pressure checked. You don't want to be fainting and falling when you're pregnant. 

Call your ob/gyn's office and talk to them.  It could be any combination of the following:
1. Dehydration
2. Low/a drop in blood pressure
3. Low blood sugar/pregnancy induced diabetes
4. Hormone induced migraine

A drop in blood pressure. It’s common for pregnant women to faint when they have never fainted before. My friend and I were in greggs one day when she was pregnant and I was choosing out a cake and my friend fell to the floor! She had never fainted before, then she fainted again a few weeks later while waiting at the dentist. She’s ok now and it’s not happened since. But it’s worth mentioning it to your dr/midwife who ever is caring for you during your pregnancy. 

You should always direct health questions to your doctor, and especially when you are pregnant. Be sure you are eating enough protein, as well as fresh fruits and vegetables. Call your doctor if things like this happen, so they can properly advise you. 

More than likely your sugar dropped. Keep a sweet or two in your purse in case it happens again. 

You're probably a bit dehydrated. Blood volume increases during pregnancy, so you have to drink more fluids than you normally would. If you need a quick pick-me-up from a dizzy spell (and don't have gestational diabetes), a sports drink or juice will give your blood sugar a boost while also getting you hydrated. 

While nausea and fatigue may have disappeared during your second trimester, by week 20 of your pregnancy some symptoms you may experience or continue experiencing include: body aches. stretch marks. skin pigmentation.

You felt faint. Hardly ground breaking 

it might mean you should go to the er

it means your vision got blurred and you got dizzy.

its not rocket science dear lol