Maybe you've done too much too soon. That's where most running aches and pains come from.
I find hip opening yoga to be very effective in keeping my hip flexors and quads loose. You can probably google or search for some exercises on you tube. If the pains anything more than muscle fatigue or soreness I'd avoid doing any stretches until you consult with a sports medicine doctor or physical therapist.
I also like my foam roller...it's basically an industrial, thicker pool noodle. You can get them at a medical supply store, or a sporting good's store. Rolling back and forth on it is essentially like giving yourself a deep tissue massage.
The main protocol is 'RICE'...Rest, Ice, Compression, Elevation. I think that in your case the first two would be easier to do. Before you ice, I would suggest slowly massaging the area to clear metabolic waste and bring new blood, oxygen and nutrients to the area. Just my opinion. I'm inclined to think that there may be an imbalance between hip flexors and the opposing muscle groups.