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Allergic reaction after a bite? [photos included]

On Saturday (6 days ago) I saw a little zit-like/blackhead-like thing behind my knee. Not thinking too much about it (have a lot of blackheads anyway so a zit here and there is nothing new to me), I scratched it, even tried to pop it if I remember correctly, a little of blood went out so I thought the business was finished.

I forgot about it and on Sunday night, I saw that a ~4cm red spot appeared in the area. Thought it was just some little allergic reaction and did nothing in particular. The next day, it started to itch, though and each day after that it stayed the same even though I've tried attacking it with either anti-insect-bite gels and OTC cortisol creams. Didn't budge as long as size is concerned.

Yesterday, however, the spot became purple instead of red and a pale-red border around it could be seen. Also, sometimes when I touch it a few times, a could feel a reaaaally mild pain. I wouldn't even describe it as pain, that's how mild it is.

However, I went to a GP and later a dermatologist - both said it wasn't looking like anything dangerous and the latter, after a close examination and interview, said it seems that the most plausible explanation was a bite by a black fly or a spider, ruling out a tick as most likely not being the case. I got a calcium twice a day prescribed, antihistamines (am taking them anyway cuase I'm allergic to pollen), a steroid-antibiotic cream once a day and oral antibiotics for three days and I'm to go for a checkup if it doesn't go away in a week from starting the treatment.

Today the pale-red border became a tad larger, it seems to have gained a centimeter or so in the direction of my foot, hasn't become wider or anything. The purple part seems not to budge. The whole thing is around 9cm in the widest point now with the purple spot measuring around 5cm. Also, the small puncture in the place where the bite most likely happened is still perfectly visible at the top of the spot.

What do you think it is? Is it dangerous? How long may it last before it disappears and why does it have this evil purple color? Smile I figured I might ask for another opinion not because I don't trust the dermatologist, but I thought it's always wise to know more opinions. Thank you a lot in advance!

Here's an album of a few close-ups and shots:
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replied August 1st, 2015
OK, a day later, the purple spot is a lot paler and the ever so slight swelling has largely subsided. What became a tad larger, though, is the pale red border of the purple spot, now it seems it went another centimeter down the leg in the widest point. It got me thinking, however - maybe it's because of the antibiotic-steroid ointment? I mean - I was spreading it both on the purple spot and the red border so I may have spread it over some healthy tissue as well and it may just be the reaction of the skin to the strong ointment which it doesn't need. This would also explain why it gained a centimeter two days in a row - it just reacted to the ointment, became red, so I used ointment on it too, then it became larger, so I used more ointment and so on.

Anyway, here's how it's looking today:

Will be grateful for any opinions Smile
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