I don't know about sexy, but I found DKNY Be Delicious and Pure both very light and have decent lasting power. My perfume with best lasting power is J'adore, it last for days if I get it on my clothe. It's a bit warm and not light, but it's not sharp so one spritz shouldn't be bad...
I know how you feel, I am super prone to migraines if I am near too strong of or too many fragrances. I like to use a little bit sweeter and light ones that are less likely to give me any trouble. Sometimes the musk and cedarwood notes bother me which may bother you too.
Try some of the CLEAN fragrances which are soft and very pleasant.
I also like the Escada scents which are sweet with a mild hint of citrus sparkle.
And I also recommend some of the Phillosophy scents like "Falling in Love" or "Amazing Grace" which are beautiful and feminine.
Try to visit your local store and smell a few different ones or try some samples!
I 100% agree with Diana regarding the philosophy perfumes - they are so light but smell amazing! The "sexiest" scent I've found from them (and wear a lot!) is "Unconditional Love." I've gotten numerous compliments about it and it is not "perfume-y" at all! Definitely pick up a sample to give it a try.
I also know exactly what you mean. I have a tendency to also get migraines easily from certain scents. I also have really bad allergies, and find more potent scents to be aggravating; especially in the morning before taking my allergy meds. Sometimes even when my allergies are at bay I just like to wear something light.
Clean, Philosophy, and Escada are all fabulous long lasting crisp scents! Clean's Outdoor Shower Fresh is what I'll typically wear if I'm running errands or hanging out casually with friends. It has a very beachy scent to it which makes it an great summer scent. My favorite Philosophy scent is Amazing Grace, which is the perfect light blend of florals and musk.
I find that Juicy Couture is another scent that is on the lighter side (which I'm wearing today). The gentle notes of vanilla, princess lilly, and creme brulee give this perfume a muted warm scent. It's soft yet very charming and romantic.
I totally agree with @dianabt and @beauty tester. I love the DKNY fragrances and the CLEAN ones are really nice too. Also try Fresh perfumes, Marc Jacobs Daisy Eau So Fresh or Clinique Happy. Ralph Lauren also makes the Big Pony collection in body sprays which are lighter than the perfumes(#2 and #4 are the most sexy,with 1 and 3 being fresher) I hope this helps and remember to get a sample or rollerball before you shell out the money for something that may give you a headache!
I absolutely love those types of perfumes. If you want a nice fruity-floral scent, try Vera Wang Lovestruck. Another reccomendation is more of a fresh scent, that would Chanel Chance Eau Tendre, or Daisy Eau So Fresh. You are also going to want to see the Nina by Nina Ricci, it is a sexy apple scent. As well, try Taylor Swift Wonderstruck. These are all slightly young, so they are better if you are a teen or early to mid twenties.
Have fun! I love fragrance shopping.
Answered[ Edited ]
I agree, I also only like really light fragrances. You already have some great recommendations. I also wanted to add a few more
Coach Poppy Flower
Caudalie Fleur de Vigne
Philosophy Pure Grace