Hi firstbub!
Don't worry, you are on the right track. One dinner out won't ruin all you've done so far!
I've seen many mums tend to fall into one of three weight loss patterns while breastfeeding babies during their first year.
1. all weight amazingly lost by about 2 months postpartum
2. weight lost by 9 months postpartum (this is probably the most common...the old saying is '9 months to get there, 9 months to get back')
3. all weight lost bar about 4-5kg, which comes off after breastfeeding finishes. It's as if a reserve is kept to ensure bfing can keep going.
Some mothers don't fit any pattern of course...
I lost all my weight by 9 months after delivery. In every case, at about 6 months quite a bit suddenly fell off. It was as if my body sensed that the baby was starting solids and decided it was OK for me to lose more weight!
If you have lost 15 kg in nearly 4 months you have done very, very well! Just continue as you are going. If at the end of bub's first year you feel that you still need to lose more, you could safely start a stricter diet. But my guess is that considering what you've lost so far, it will all come off easily over the next 6 months.