If your baby is still gaining weight, and is healthy, perky, nappies as they should be, then I wouldn't worry at all - and i certainly wouldn't force feed her! that's a terrible suggestion!
DS was always a 'skinny' baby, he never ever had 'chubby legs' He was always low on the weight centiles, but if your baby is anywhere on the chart, then it's fine. If your baby puts no weight on at all over a few weeks, or loses weight, then that would be worrying.
Are you or her dad reasonably slight in build? Is she caucasion? (DS is half asian, and i found out after weeks of wory - from a MN-er of course - that the charts are based on bottlefed caucasion babies, who tend to be haevier than asian babies).
Also - is your baby becoming more active? Are you playing 'sitting up' games with her? It all takes energy so she might be having a period where her weight increase slows down a bit.
Force feed her indeed!