What kind of soreness does she have?
Is it internal - like from you bumping her cervix during thrusting? If it's this kind of soreness, then you can try different kinds of positions so that her cervix doesn't get bumped. (If she has continued internal pain she should see an ob-gyn to rule out any other potential conditions, such as cysts, fibroids, endometriosis, etc.)
Is it muscular - like she is using some muscles that she hasn't been for awhile? If it's muscular, then continuing to have sex and engaging in additional exercise to boost her muscle strength and energy may help.
Is it at the opening - from a lack of lubrication or even just not being used to having sex as often or having something of the girth that you are (you mentioned that she is still very tight)? For this, you can add lubrication - you may have to experiment or ask her doctor for suggestions if she has a lot of sensitivity. If it's your girth, then making sure she is very aroused is helpful, or she could work with vaginal dilators. If she's had children and had an episiotomy or tearing and the scars are bothering her, she should see her ob-gyn for options.
If she has continued pain or soreness no matter what you do or try, then she should make a trip in to see her ob-gyn and talk about it. Most women will have pain or soreness during intercourse or afterward at least once in their lives.
There can be all kinds of causes - some mild and easy to treat or recover from, and some not so.