Ugh I had a whole post on this yesterday, but the mods took it down because I posted episode-related content too soon. Check out the photos u/GuniGooGoo added in the comments of that post to see what I'm referencing:
Heather's black eye reveal took place the morning of day 3 of the trip where we see a deep black bruise and heavy swelling. The episode is edited to suggest that Heather received the bruise in the early hours of day 3. I think this is a misdirection.
Deep bruises like Heather's take 1-2 days to form. After blood vessels break and blood pools to the site of the injury, the blood slowly oxidizes over this length of time, which is what gives a bruise it's color. It is unlikely that a bruise as severe as Heather's took less than a day to form.
I believe Heather had this bruise as early as Night 1. When Heather shows up for the dinner, the area around her left eye looks purple. "But OP... Perhaps this is her eyeshadow?" Take a look at Episode 10 at 28:47. You can see light purple on the outside and bottom of Heather's left eye socket (aka her left zygomatic bone) in an area that extends way behind where eyeshadow would even drift to. Shortly thereafter, she puts on her sunglasses. At night. In the middle of dinner.... what?! I think someone alerted her that her make up is starting to fade and, since she didn't have time in the middle of the argument to touch it up, she puts on her sunglasses.
Other than when the ladies wake up on day 2, Heather is seen wearing sunglasses all day, even when she is in the bubble ball. Dana bumps up hard against her, and Heather plops down, looking stunned. In my opinion, the handles of the ball hit against Heather's sunglasses and aggravated an already existing bruise, and the pain shocked her.
On the night of day 2, Heather's wig seems to comb over her left eye wherever possible, although sometimes the combing over looks awkward. Throughout the dinner, the area under her left eyebrow looks puffier than the area under her right eyebrow. Check out 40:54 and 42:17 of Episode 11. Heather's make up at the bar after the dinner looks fresh like she touched it up. We don't get a good view of her face for the rest of the night when they are at the club or back at the house.
By the time day 3 rolls around, the black eye is too dark and swollen to hide underneath any make up or sunglasses. So, Heather/production spun it into a storyline.
Jen, Meredith, and Angie K, who stayed up with Heather, would have gotten a closer view of her eye in the early hours of day 3. I think that is why they didn't seem as shocked as the other girls when Heather revealed her eye the next morning.
As for the scratches... Heather has pointy, almond shaped nails. The scratches are closer to her armpit area where a tube top would sit. I think she scratched herself when she was pulling her jumpsuit top down to flash her boobs throughout the night. The lipstick on her arm also more closely matches the shade she was wearing the night before than anyone else's.
TLDR; Heather would have to have received her black eye on day 1 or early day 2 for it to look that bad on the morning of day 3. There are signs of the bruise as early as night 1. I think some of her friends or production knew about the bruise as early as that and spun it into a storyline when the bruise got so bad that it couldn't be covered.