Direct sunlight, as well as being close to bright indoor lighting (no matter the time of year) will, of course, cause this to happen faster, but the same will happen if you don't keep you juice in a cool, dark place summer, winter - all year.
I keep mine in the warm part of the refrigerator during the summer and actually drip from larger bottles into the very small sample bottles i have kept and cleaned so I only go out with the amount I need and not risk ruining a whole bottle in the summer heat. What happens in heat and faster in sunlight, is that much of the PG/VG can evaporate away and leave you with nicotine that is not diluted as much as it had been before. Since your symptoms are those of someone with nicotine poisoning it sounds like that day in the sun cause the juice to end up very strengthened. 24 mg after a few hours in the sun can be anywhere from 30-40 mg depending on how much evaporates and if you are not used to that strength and vape it as much as you do your usual, low strength, you can end up being pretty sick.
VG can go bad, but its usually a risk of that happening when you buy large amounts of ml bottles to make your own and keep it on the self too long and then have to deal with mold.
This is a good warning for everyone, especially for those living in parts of the world that have very long, hot, dry summers but applies to anyone who subjects their juice to warm temps for a while. If you leave a bottle in the heat for what you think might be too long and worry you now have strengthened the juice, check the bottle carefully and look for sighs of evaporation, such as 'sweating' on the inside of the bottle. Don't take a chance once you see a lot of water evaporated out of the juice (shaking it and getting the water drops mixed back in will not 'save' it as many say it will as what evaporates is the water mixed in with the nicotine itself and you would need to reprocess the nicotine to get the water mixed back in, shaking it to 'reabsorb' the water just mixes water into the glycerin used in the juice).
If this happens, especially with the higher level nicotine juice its probably not worth keeping it around - toss it and buy fresh. It may be less of a risk if you are using a low mg of nicotine and the bottle only shows a bit of sweat - most likely you have only just strengthened it to around the next level - or that's the rule of thumb. It may taste differently to you after it has evaporated in a bottle - the flavor may taste a bit diluted.
So, when going out, make it a practice to use smaller bottles or prefill clearos/carto tanks with what you need. As I mentioned before, using cleaned and well dried empty sample bottles (or even large cleaned and dried bottles when you have used up the e-liquid) make it easy to just take what you know you will vape along with you and not you entire supply. Keep what you carry away from your body (unless you are in freezing weather in which you want to keep it from freezing) and in a dark place. Juice in dark bottles will also lessen the effects or bright indoor light. Pocketbooks/purses'/backpacks/briefcases/luggage are all decent places while a clothing pocket can get way too warm even if it shades your juice.
This is not all said to convince anyone vaping is 'dangerous' but like anything whether its tonight's dinner, or medication or other things you use internally, safe handling is the best practice. People tend to very careful with hardware to make sure its in perfect working order, or properly cleaned or tested well when its rebuilt and so on but very often neglect to correctly handling their juices and either ignore or do not know the correct handling of juice.