Jami,
You might have a look at the Eagle Creek line of Backpacks, as they have numerous models in various sizes, some of which are specially tailored for women. Given your size and weight, a medium sized model is a good place to start, however be sure to pack light!
Note that if you'll be flying with budget airlines in Europe, their luggage requirements are much stricter, so you may be forced to check your Bag.
Some of the features to look for would be a "stowable harness" (stow the straps behind a zippered panel during air travel), a detachable Daypack (if you have to check the main pack, that can be used for carry-on), and panel-load design (with zippers which can be locked - provides at best a minimal level of security).
However the MOST IMPORTANT feature is to be fitted properly according to your torso size. Most of the weight should rest on your hips rather than your shoulders. Given the various methods you'll be using for travel, there could be situations where you'll be wearing the pack for longer periods, so good fit is important!
You might also check Rick's Travel store on this website, as one of the bags there may also fit your needs. The Convertible Backpack is supposedly small enough to meet carry on requirements, but I suspect if this was fully packed, the size or weight would required it to be checked.
If you want a shoulder bag that can also function as a Backpack, you might check the Tom Bihn (Tri-Star) or Red Oxx (Air Boss) websites as they have very well built products that might fit your needs.
Happy travels!