indian runners egg laying

[norris duck]

i have a 2 Indian runners a female Vera duck worth and a drake called duck Norris. i am new to caring for ducks and would like some simple egg laying answers. i have been all over the Internet and just cant get a straight answer. my female duck laid her first egg on Easter day she continued to lay for about 9 days but then just stopped for about 3 weeks, she then laid a tiny pebble shaped egg then nothing again till last week but the egg was dark green with white lumps of chalky stuff on the outside. when i cracked the egg the yolk was also a dark green stony colour. she hasn't laid an egg since i am really worried about her as physically she looks fine i have got her on a natural garden diet with some layers pellets and dark greens for vitamins can anybody help me please. also do ducks have a egg laying season or do they just continuously keep laying?.