Most bf babies level out on the growth charts at this age, the charts are based on ff babies, they gain weight at a more regular rate, but bf babies don't. As long as she is gaining weight and not losing it don't be alarmed by drop down the centiles. Trust your eyes, is she chubby? Healthy? Alert? Happy? Gaining weight (even if it's slowly)? If the answer to these questions is yes then just chill and carry on doing the fabulous job you currently are doing!
There is a growth spurt at about 18/20 weeks, it can be fierce, but it is a growth spurt, and not an indication of needing solids or formula, your milk is still the most calorie dense food there is, and all she needs for the next 6 months, solids are not actually a significant source of calories until babies are over a year old!
Please don't feel that what you are expressing is an indication of how much milk you make, it really isn't, try not to be discouraged!!
If she is really interested in food, wedge her in a high chair, sitting upright, and put some cooked broccoli florets or chunky (cooked) carrot sticks on the tray and let her try to feed herself. If she really is ready for/in need of solids she'll be able to pick it up and put it into her mouth and gum/bite bits off and chew them. See this and this.